Where the concept of evolution proved itself particularly useful was in the interpretation of structures which were not immediately conditioned by adaptation to present requirements, such as, for instance, the arrangement of gill-slits and aortic arches in the foetus of land Vertebrates. Such "heritage characters" could only be explained on the hypothesis that they had once had functional or adaptational meaning. Why, for instance, should the blastopore so often appear as a long slit, closing by concrescence, unless this had been the original method of its formation in remote Coelenterate ancestors?

The point hardly requires elaboration, since it has become an integral part of all our thinking on biological problems. It may be as well, however, for the sake of continuity, to give one or two examples of the historical interpretation of animal structures. The first may conveniently be the phylogenetic interpretation of the contrast between "membrane" and "cartilage" bones.

In his Grundzuege of 1870, Gegenbaur made the suggestion that the investing or membrane bones were derived phylogenetically from integumentary ossifications, and this was worked out in detail a few years later by O. Hertwig.[458]

Many years before, several observers—J. Mueller, Williamson, and Steenstrup—had been struck with the resemblance existing between the placoid scales and the teeth of Elasmobranch fishes. Hertwig followed up this clue, and came to the conclusion not only that placoid scales and teeth were strictly homologous, but also that all membrane bones were derived phylogenetically from ossifications present in the skin or in the mucous membrane of the mouth, just as cartilage bones were derived from the cartilaginous skeletons of the primitive Vertebrates. In some cases this manner of derivation could even be observed in ontogeny, as Reichert had seen in the Newt, where certain bones in the roof of the mouth are actually formed by the concrescence of little teeth, (supra, p. 163). Hertwig considered that the following bones were originally formed by coalescence of teeth—parasphenoid, vomer, palatine, pterygoid, the tooth-bearing part of the pre-maxillary, the maxillary, the dentary and certain bones of the hyo-mandibular skeleton of Teleosts. All the investing bones (Deckknochen) of the skull were of common origin, and could be traced back to integumentary skeletal plates, which in the ancestral fish formed a dense carapace.

These conclusions were accepted by Koelliker himself, who wrote in his Entwickelungsgeschichte (1879)—"The distinction between the primary or primordial, and the investing or secondary bones is from the morphological standpoint sharp and definite. The former are ossifications of the (cartilaginous) primordial skeleton, the latter are formed outside this skeleton, and are probably all ossifications of the skin or the mucous membrane" (p. 464).

Gegenbaur[459] consistently upheld the phylogenetic derivation of investing bones from dermal ossifications, and even went further and derived substitutionary bones as well from the integument, thus establishing a direct comparison between the skeletal formations of Vertebrates and Invertebrates. Investing bones were actual integumentary ossifications which had gradually sunk beneath the skin to become part of the internal skeleton; substitutionary bones were produced by cells (osteoblasts) which were ultimately derived from the integument.[460]

A further instance of the historical interpretation of animal structure, taken from quite a different field, is afforded by the speculations of Dollo[461] on the ancestral history of the Marsupials. In a brilliant paper of 1880[462] Huxley made the suggestion that the ancestors of Marsupials were arboreal forms. "I think it probable," he wrote, "from the character of the pes, that the primitive forms, whence the existing Marsupialia have been derived, were arboreal animals; and it is not difficult, I conceive, to see that, with such habits, it may have been highly advantageous to an animal to get rid of its young from the interior of its body at as early a period of development as possible, and to supply it with nourishment during the later periods through the lacteal glands, rather than through an imperfect form of placenta" (p. 655). Dollo followed up this suggestion, which had in the meantime been strengthened by Hill's discovery of a true allantoic placenta in Perameles, by demonstrating in the foot of present-day Marsupials certain features which could only be interpreted as inherited from a time when the ancestors of Marsupials were tree-living animals. These were the occurrence of an opposable big toe (when this was present at all), the great development of the fourth toe, the reduction and partial syndactylism of the second and third toes, and in some cases the regression of the nails. These characters were shown to be typical of arboreal Vertebrates, and their occurrence in forms not arboreal indicated that these were descended from tree-living ancestors. Traces of an arboreal ancestry could be demonstrated even in the marsupial mole Notoryctes.

These are only two examples out of hundreds that might be given. Present day structure was interpreted in the light of past history; the common element in organic form was seen to be due to common descent; the existence of vestigial and non-functional organs was no longer a riddle.

There was even a tendency to concentrate attention upon the historical side of structure, upon what the animal passively inherited rather than upon what it personally achieved. Homologies were considered more interesting than analogies, vestigial organs more interesting than foetal and larval adaptations. Convergence was anathema. The dead-weight of the past was appreciated at its full and more than its full value; and the essential vital activity of the living thing, so clearly shown in development and regeneration, was ignored or forgotten.

But evolutionary morphology for all practical purposes was a development of pure or idealistic morphology, and was powerless to bring to fruit the new conception with which evolution-theory had enriched it. The reason is not far to seek. Pure morphology is essentially a science of comparison which seeks to disentangle the unity hidden beneath the diversity of organic form. It is not immediately concerned with the causes of organic diversity—that is rather the task of the sciences of the individual, heredity and development. To take an example—the recapitulation theory may legitimately be used as a law of pure morphology, as stating the abstract relation of ontogeny to phylogeny, and the probable line of descent of any organism may be deduced from it, as a mere matter of the ideal derivation of one form from another; but an explanation of the reason for the recapitulation of ancestral history during development can clearly not be given by pure morphology unaided. From the fact that the common starfish shows in the course of its development distinct traces of a stalk[463] it is possible to infer, taking other evidence also into consideration, that the ancestors of the starfish were at one stage of their existence stalked and sessile organisms. But this leaves unanswered the question as to how and why the starfish does still repeat after so many millions of years part of the organisation of one of its remote ancestors. Why is this feature retained, and by what means has it been conserved through countless generations? It is clear that the answer can be given only by a science of the causes of the production and retention of form, by a causal morphology, based upon a study of heredity and development.

From the point of view of the pure morphologist the recapitulation theory is an instrument of research enabling him to reconstruct probable lines of descent; from the standpoint of the student of development and heredity the fact of recapitulation is a difficult problem whose solution would perhaps give the key to a true understanding of the real nature of heredity.

To make full use of the conception of the organism as an historical being it is necessary then to understand the causal nexus between ontogeny and phylogeny.

We shall see in the next chapter that the transformation of morphology from a comparative to a causal science did take place towards the end of the century, and that some progress was made towards an understanding of the relation between individual development and ancestral history, particularly by Roux and Samuel Butler, working with the fruitful Lamarckian conception of the transforming power of function.

[456] The importance of convergence came to be realised after the vogue of phylogenetic speculation had passed—see Friedmann, Die Konvergenz der Organismen, Berlin, 1904, and A. Willey, Convergence in Evolution, London, 1911. Also L. Vialleton, Elements de morphologie des Vertebres, Paris, 1912.

[457] From this point of view there is a very profound analogy between artificial and natural selection. Upon the theory of natural selection organisms are lifeless constructs which are mechanically perfected by external agency, just as machines are improved by a process of conscious selection of the most successful among a number of competing models. (Cf. passage quoted below, on p. 308.)

[458] Arch. f. mikr. Anat., xi. (suppl.), 1874; Morph. Jahrb., ii., 1876, v. 1879, and vii., 1882.

[459] Vergleich. Anat. d. Wirbelthiere, i., pp. 200-1, 1898.

[460] For a full historical account of work on membrane and cartilage bones (as well as on the theory of the skull) see E. Gaupp, "Altere und neuere Arbeiten ueber den Wirbelthierschaedel," Ergeb. Anat. Entw., x., 1901, and "Die Entwickelung des Kopfskelettes," in Hertwig's "Handbuch vergl. exper. Entwickelungslehre d. Wirbelthiere," iii., 2, pp. 573-874, 1905.

[461] "Les Ancetres des Marsupiaux etaient-ils arboricoles?" Trav. Stat. zool. Wimereux, vii., pp. 188-203, pls. xi.-xii., 1899. See also Bensley, Trans. Linn. Soc. (2) ix., pp. 83-214, 1903.

[462] Proc. Zool. Soc., pp. 649-62, 1880. Sci. Mem., iv., pp. 457-72.

[463] J. F. Gemmill, Phil. Trans. B, ccv., p. 255, 1914.



CHAPTER XVIII

THE BEGINNINGS OF CAUSAL MORPHOLOGY

Until well into the 'eighties animal morphology remained a purely descriptive science, content to state and summarise the relations between the coexistent and successive form-states of the same and of different animals. No serious attempt had been made to discover the causes which led to the production of form in the individual and in the race.

It is true that evolution-theory had offered a simple solution of the great problem of the unity in diversity of animal forms, but this solution was formal merely, and went little beyond that abstract deduction of more complex from simpler forms, which had been the main operation of pre-evolutionary morphology. Little was known of the actual causes of ontogeny, and nothing at all of the causes of phylogeny; it was, for instance, mere rhetoric on Haeckel's part to proclaim that phylogeny was the mechanical cause of ontogeny.

Animal physiology, on its side, had developed in complete isolation from morphology into a science of the functioning of the adult and finished animal, considered as a more or less stable physico-chemical mechanism. Since the days of Ludwig, Claude Bernard and E. du Bois Reymond, the physiologists' chief care had been to analyse vital activities into their component physical and chemical processes, and to trace out the interchange of matter and energy between the organism and its environment. Physiologists had left untouched, perhaps wisely, the much more difficult problem of the causes of the development of form. For all practical purposes they took the animal-machine as given, and did not trouble about its mode of origin. They held indeed that form-production was due to a complex of physico-chemical causes, which they hoped some day to unravel;[464] but this future physiology of development remained quite embryonic.

Physiology then had not really come into contact with the problems of form, and it could give the morphologist no direct help when he turned to investigate the causes of form-production. It had, however, a determining influence upon the methods of those who first broke ground in this No Man's Land between morphology proper and physiology. But it is significant that it was a morphologist and not a physiologist that did the first spade-work.

The pioneer in this field, both as investigator and as thinker, was W. Roux, who sketched in the 'eighties the main outlines of a new science of causal morphology, to which he gave the name of Entwicklungsmechanik. The choice of name was deliberate, and the word implied, first, that the new science was essentially an investigation of the development of form, not of the mode of action of a formed mechanism, and second, that the methods to be adopted were mechanistic.[465]

Though Roux was the only begetter of the science of Entwicklungsmechanik, he was, of course, not the first to investigate experimentally the formative processes of animal life. Study of regeneration dates back to Trembley (1740-44), Reaumur (1742), Bonnet (1745), and Spallanzani (1768-82),[466] and in the years preceding Roux's activity good work was done by Philipeaux. A beginning had been made with experimental teratology by E. Geoffroy St Hilaire and others, and the work of C. Dareste[467] remains classical. Back in the 18th century, some of John Hunter's experiments had a bearing upon the problems of form; his work on transplantation was followed up in the 19th century by Flourens, P. Bert, Ollier and many others. In founding in 1872 the Archives de Zoologie experimentale et generale H. de Lacaze-Duthiers put forward in his introduction a powerful plea for the use of the experimental method in zoology.

In some ways more directly connected with Entwicklungsmechanik was His's attempt in 1874[468] to explain on mechanical principles the formation of certain of the embryonic organs by the bendings and foldings of tubes or plates of cells. "His compared the various layers of the chick embryo to elastic plates and tubes; out of these he suggested that some of the principal organs might be moulded by mere local inequalities of growth—the ventricles of the brain, for instance, the alimentary canal, the heart—and he further succeeded in imitating the formation of these organs by folding, pinching, and cutting india-rubber tubes and plates in various ways."[469]

But Roux was undoubtedly the first to make a systematic survey of the problems to be solved and to work out an organised method of attack. His earliest work deals with the important problem of functional adaptation—its importance to the organism, and its possible mechanistic explanation. The first paper[470] was a study of the branching and distribution of the arteries in the human body (1878), and a second paper on the same subject followed in 1879.[471]

In these papers Roux showed how the development of the blood-vascular system was largely determined by direct adaptation to functional requirements, and he inferred the existence in the vascular tissues of certain vital properties, in virtue of which the functional adaptation of the blood-vessels came about. Thus the intima or inner lining must possess the faculty of so reacting to the friction set up by the blood-current as to oppose the least possible resistance to its flow; the muscular coats must react to increased pressure by growing thicker, and so on.

These papers were followed in 1881 by his well-known book, Der Kampf der Theile im Organismus, which contained the working-out of his mechanistic explanation of functional adaptation, and most of the elements of his general "causal-analytical" theory of form production. The significance of the book was popularly considered at the time to lie in its supposed application of the selection idea to the explanation of the internal adaptedness of animal structure—in the theory of "cellular selection," and the book owed its success to its fitting in so well with the prevalent Darwinism of the day. But its real importance, as a big step towards causal morphology, was naturally not so fully appreciated.

During the next few years Roux continued his studies on functional adaptation,[472] and at the same time made a new departure by inaugurating, almost contemporaneously with the physiologist Pflueger, the study of experimental embryology. Isolated observations had previously been made upon the development of single blastomeres or parts of blastulae, by Haeckel and Chun for instance,[473] but Roux[474] and Pflueger[475] were the first to investigate the subject systematically, choosing for their work the egg of the frog.[476] Roux continued for many years to follow up this line of work.[477]

In 1890 he drew up a programme and manifesto[478] of Entwicklungsmechanik as "an anatomical science of the future," and in 1895 he founded the famous Archiv fuer Entwicklungsmechanik,[479] publishing in the same year the two large volumes of his collected papers,[480] of which the first volume dealt with functional adaptation, the second with experimental embryology.

His subsequent work includes several important general papers;[481] besides a number of special memoirs dealing with the factors of development, and with his original subject, functional adaptation.[482]

In our sketch of his views we shall have occasion to refer particularly to his publications of 1881, 1895 (the Einleitung), 1902, 1905, and 1910.

Although Roux's biological philosophy is out-and-out mechanistic, he yet recognises the difficulty, even the impossibility, of straightway reducing development to the physico-chemical level. He tries to steer a course midway between the simplicist conceptions of the materialists and the "metaphysics" of the neo-vitalist school, which the experimental study of development and regeneration soon brought into being. In 1895 he writes:—"The too simple mechanistic conception on the one hand, and the metaphysical conception on the other represent the Scylla and Charybdis, between which to sail is indeed difficult, and so far by few satisfactorily accomplished; it cannot be denied that with the increase of knowledge the seduction of the second has lately notably increased" (p. 23).

The via media adopted by Roux is the analysis of development, not directly into simple physico-chemical processes, but into more complex organic processes dependent upon the fundamental properties of living matter. The aim of Entwicklungsmechanik is defined by Roux to be the reduction of developmental events to the fewest and simplest Wirkungsweisen, or causal processes.[483] Two classes of causal processes may be distinguished, as "complex components" and "simple components" of development. The latter are directly explicable by the laws of physics and chemistry; the former, while in essence physico-chemical, are yet so very complicated that they cannot at present be reduced to physico-chemical terms. The ultimate aim of Entwicklungsmechanik is to reduce development to its "simple components," but its main task at the present day and for many years to come is the analysis of development into its "complex components."

These complex components must be accepted as having much of the validity of physical and chemical laws. They are mysterious in the sense that they cannot yet be explained mechanistically, but they are constant in their action, and under the same conditions produce always the same effect—hence they may be made the subject of strictly scientific study. They represent biological generalisations, in their way of equal validity with the generalisations of physics and chemistry.

The principal "complex components" which Roux recognises are somewhat as follows:—First come the elementary cell-functions of assimilation and dissimilation, growth, reproduction and heredity, movement and self-division (as a special co-ordination of cell-movements). Then at a somewhat higher level, self-differentiation, and the trophic reaction to functional stimuli. Components of even greater complexity may also be distinguished, as, for instance, the biogenetic law. The various tropisms exhibited in development may be regarded as "directive" complex components. There must be added, not as being itself a component, but rather as a mode or peculiar property of all functioning, the omnipresent faculty of self-regulation.

It will be noticed that Roux's "complex components" are simply the general properties or functions of organised matter.

Expressing Roux's thought in another way, we might say that life can only be defined functionally, i.e., by an enumeration of the "complex components" or elementary functions which all living beings manifest, even down to the very simplest. "Living beings," writes Roux, "can at present be defined with any approach to completeness only functionally, that is to say, through characterisation of their activities, for we have an adequate acquaintance with their functions in a general way, though our knowledge of particulars is by no means complete" (p. 105, 1905). Defined in the most general and abstract way, living things are material objects which persist in spite of their metabolism, and, by reason of their power of self-regulation, in spite also of the changes of the environment. This is the "functional minimum-definition of life" (pp. 106-7, 1905).

We may now go on to consider the relation of function to form throughout the course of development. Roux distinguishes in all development two periods, in the first of which the organ is formed prior to and independent of its function, while in the second the differentiation and growth of the organ are dependent on its functioning. Latterly (1906 and 1910) Roux has distinguished three periods, counting as the second the transition period when form is partly self-determined, partly determined by functioning. As this conception of Roux's is of the greatest importance we shall follow it out in some detail.

The idea was first elaborated in the Kampf der Theile (1881), where he wrote:—"There must be distinguished in the life of all the parts two periods, an embryonic in the broad sense, during which the parts develop, differentiate and grow of themselves, and a period of completer development, during which growth, and in many cases also the balance of assimilation over dissimilation, can come about only under the influence of stimuli" (p. 180). There is thus a period of self-differentiation in which the organs are roughly formed in anticipation of functioning, and a period of functional development in which the organs are perfected through functioning and only through functioning. The two periods cannot be sharply separated from one another, nor does the transition from the one to the other occur at the same time in the different tissues and organs.

The conception is more fully expressed in 1905 as follows:—"This separation (of development into two periods) is intended only as a first beginning. The first period I called the embryonic period [Greek: kat' exochen] or the period of organ-rudiments. It includes the 'directly inherited' structures, i.e., the structures which are directly predetermined in the structure of the germ-plasm, as, for instance, the first differentiation of the germ, segmentation, the formation of the germ-layers and the organ-rudiments, as well as the next stage of 'further differentiation,' and of independent growth and maintenance, that is, of growth and maintenance which take place without the functioning of the organs.

"This is accordingly the period of direct fashioning through the activity of the formative mechanism implicit in the germ-plasm, also the period of the self-conservation of the formed parts without active functioning.

"The second period is the period of 'functional form-development.' It includes the further differentiation and the maintenance in their typical form of the organs laid down in the first period; and this is brought about by the exercise of the specific functions of the organs. This period adds the finishing touches to the finer functional differentiation of the organs, and so brings to pass the 'finer functional harmony' of all organs with the whole. The formative activity displayed during this period depends upon the circumstance that the functional stimulus, or rather the exercise by the organs of their specific functions, is accompanied by a subsidiary formative activity, which acts partly by producing new form and partly by maintaining that which is already formed.... Between the two periods lies presumably a transition period, an intermediary stage of varying duration in the different organs, in which both classes of causes are concerned in the further building-up of the already formed, those of the first period in gradually decreasing measure, those of the second in an increasing degree" (pp. 94-6, 1905).

In the first period the organ forms or determines the function, in the second period the function forms the organ, or at least completes its differentiation. It is characteristic that in the first period functionally adapted structure appears in the complete absence of the functional stimulus.

The explanation of the difference between the two periods is to be found in the different evolutionary history of the characters formed during each. First-period characters are inherited characters, and taken together constitute the historical basis of the organism's form and activity; second-period characters are those of later acquirement which have not yet become incorporated in the racial heritage.

Inherited characters appear in development in the absence of the stimulus that originally called them forth; acquired characters are those that have not yet freed themselves from this dependence upon the functional stimulus. First-period characters were originally, like second-period characters, entirely dependent for their development upon the functional stimuli in response to which they arose, and only gradually in the course of generations did they gain that independence of the functional stimulus which stamps them as true inherited characters. Speaking of the formative stimuli which are active in second-period development, Roux writes:—"These stimuli can also produce new structure, which if it is constantly formed throughout many generations finally becomes hereditary, i.e., develops in the descendants in the absence of the stimuli, becomes in our sense embryonic" (p. 180, 1881). Again, "form-characteristics which were originally acquired in post-embryonic life through functional adaptation may be developed in the embryo without the functional stimulus, and may in later development become more or less completely differentiated, and retain this differentiation without functional activity or with a minimum of it. But in the continued absence of functional activity they become atrophied ... and in the end disappear" (p. 201, 1881).

This conception of the nature of hereditary transmission is an important one, and constitutes the first big step towards a real understanding of the historical element in organic form and activity. It supplies a practical criterion for the distinguishing of "heritage" characters from acquired characters, of palingenetic from cenogenetic—a criterion which descriptive morphology was unable to find.[484] The introduction of a functional moment into the concept of heredity was a methodological advance of the first importance, for it linked up in an understandable way the problems of embryology, and indirectly of all morphology, with the problem of hereditary transmission, and gave form and substance to the conception of the organism as an historical being.

It is this element in Roux's theories that puts them so far in advance of those of Weismann. Weismann did not really tackle the big problem of the relation of form to function, and he left no place in his mechanical system of preformation for functional or second-period development; he conceived all development to be in Roux's sense embryonic, and due to the automatic unpacking of a complex germinal organisation. Roux himself was to a certain extent a preformationist, for the development of his first-period characters is conditioned by the inherited organisation of the germ-plasm, and is purely automatic. It was indeed his experiments on the frog's egg (1888) that supplied some of the strongest evidence in favour of the mosaic theory of development. The number of Anlagen which he postulates in the germ is however small, and the germ-plasm in his conception of it has a relatively simple structure (p. 103, 1905).

The transmission of acquired characters forms, of course, an integral part of Roux's conception of heredity and development, for without this transmission second-stage characters could not be transformed into first-stage characters. He discusses this difficult question at some length in the Kampf der Theile, coming to the conclusion that such transmission takes place in small degree and gradually, and that many generations are required before a new character can become hereditary. He thinks that acquired characters are probably transmitted at the chemical level. It is conceivable that acquired form-changes are dependent on chemical changes, or are correlative with such, and that, since the germ-cells stand in close metabolic relations with the soma, these chemical changes may soak through to the germ-cells and so modify them that a predisposition will appear in the descendants towards similar form-changes.[485] From this point of view the problem of transmission might be merged in the broader problem of the production of form through chemical processes—the central problem of all development.

Inherited characters develop by an automatic process of self-differentiation, and the separate parts of the embryo show during this first period a surprising functional independence of one another. But this state of things changes progressively as the second period is reached, until finally all form-production and maintenance and all correlation depend upon functioning. It is in the first period of automatic development through internal "determining" factors that the "developmental" functions in the strict sense, e.g. automatic growth, division and self-differentiation, are most clearly shown. In the second or "functional" period the formative influence of function upon structure comes into play, and development becomes largely a matter of "functional adaptation" to functional requirements.

All structure, according to Roux, is either functional or non-functional. The former includes all structure that is adapted to subserve some function. "Such 'functional structures' are, for example, the composition of striated muscle fibres out of fibrillae and these out of muscle-prisms, or again the length and thickness of the muscles, the static structure of the bones, the composition of the stomach and the blood-vessels out of longitudinal and circular fibres, the external shape of the vertebral centra and of the cuneiform bones of the foot" (p. 73, 1910). Indeed, as Cuvier had already pointed out, practically every organ in the body shows a functional structure which is accurately and minutely adjusted to the function it is intended to perform. Thus, to take some further examples, the arteries are admirably adapted as regards size of lumen, elasticity of wall, direction of branching, to conduct the blood to all parts of the body with the least possible waste of the propelling power through frictional resistance. So, too, the spongy substance of the long bones is arranged in lamellae which take the direction of the principal stresses and strains which fall upon the bones in action.

Functional structure may be formed either in the first or in the second period of development, may be either inherited or acquired, but it reaches its full differentiation only in the second period, i.e., under the influence of functioning. Practically speaking, functional structure is directly dependent for its full development and for its continued conservation upon the exercise of the particular function which it serves. In the second period, but not in the first, increased use leads to hypertrophy of the functional structure, disuse to atrophy.

From functional structure is to be distinguished nonfunctional structure, which has no relation to the bodily functions—is neither adapted to perform any of these, nor has arisen as a by-product of functional activity. "To this category belong, for example, among typical structures, the triangular form of the cross-section of the tibia, the dolicocephalic or brachycephalic shape of the skull, most of the external characters distinguishing genera and species, many of the external features of the embryo which change in the course of development, besides most of the abnormal forms shown by monstrosities, tumours, etc." (p. 74, 1910). Non-functional structure is not affected by functional adaptation, and may accordingly be left out of consideration here.

Now the influence of functioning upon the form and structure of an organ is twofold. There is first the immediate change brought about by the very act of functioning—for example, the shortening and thickening of skeletal muscles when they act. This is a purely temporary change, for the organ at once returns to its normal quiescent state as soon as it ceases to function. Such temporary functional change, brought about in the moment of functioning, is usually dependent for its initiation upon some neuro-muscular mechanism, though it may be elicited also by a chemical stimulus. It is thus always a phenomenon of "behaviour." "From such temporary changes are sharply to be distinguished all permanent alterations which first appear in perceptible fashion through oft-repeated or long-continued, enhanced functional activity. These produce a new and lasting internal equilibrium of the organ, consisting in an insertion of new molecules or a rearrangement of old. For this reason they outlast the periods of functional form-change, or, if as in the case of the muscles they themselves alter during functional activity, they regain their state when the organ ceases to function" (p. 72, 1910). "Oft-repeated exercise or heightened exercise of the specific functions, or repeated action of the functional stimuli which determine them, produces, as we have said before, true form-changes as a by-product. These are of two kinds. In so far as these form-changes facilitate the repetition of the specific functions, I have called them functional adaptations.... Such as do not improve the functioning of the organ are indeed by-products of functioning, but without adaptive character; they do not belong to the class of functional adaptations at all" (p. 75, 1910).

We may now enquire in what way functional adaptations can arise as by-products of functioning.

It is clear that natural selection in the sense of individual or "personal" selection cannot adequately explain the origin of functional structure and the functional harmony of structure, for thousands of cells would have to vary together in a purposive way before any real advantage could be gained in the struggle for existence, and it is in the highest degree unlikely that this should come about by chance variation.[486] The development of purposive internal structure is only to be explained by the properties of the tissues concerned.

In illustration and proof of the statement that functional adaptation is due to the properties of the tissues we may adduce the development and regulation of the blood-vascular system, which has been thoroughly studied from this point of view by Roux and Oppel (1910).

It appears that only the very first rudiments of the vascular system are laid down in the short first period of automatic non-functional development. All the subsequent growth and differentiation of the blood-vessels falls into the second period, and is due wholly or in great part to direct functional adaptation to the requirements of the tissues. Thus from the rudiments formed in the first period there sprout out the definitive vessels in direct adaptation to the food-consumption of the tissues they are to supply. The size, direction and intimate structure of these vessels are accurately adjusted to the part they play in the economy of the whole, and this adjustment is brought about in virtue of the peculiar properties or reaction-capabilities of the different tissues of which the blood-vessels are composed.

The properties which Roux finds himself compelled to postulate in the vascular tissues, after a thorough-going analysis of the different kinds of functional adaptation shown by the blood-vessels, are summarised by him as follows:—

"(1) The faculty—depending on a direct sensibility possessed by the endothelium and perhaps also by the other layers of the intima—of yielding to the impact of the blood, so far as the external relations of the vessel permit. In this way the wall adapts itself to the haemodynamically conditioned 'natural' shape of the blood-stream, and reaches this shape as nearly as possible." Through this faculty of the lining tissue of the blood-vessels, the size of the lumen and the direction of branching are so regulated as to oppose the least possible resistance to the flow of the blood.

"(2) The faculty possessed by the endothelium of the capillaries of each organ of adapting itself qualitatively to the particular metabolism of the organ." This adaptedness of the capillaries is, however, more usually an inherited state, i.e., brought about in the first period of development.

"(3) The faculty possessed by the capillary walls of being stimulated to sprout out and branch by increased functioning, i.e., by increased diffusion, and their power to exhibit a chemically conditioned cytotropism, which causes the sprouts to find one another and unite. A similar process can be directly observed in isolated segmentation-cells, which tend to unite in consequence of a power of mutual attraction.

"(4) The faculty of developing normal arterial walls in response to strong intermittent pressure, and normal venous walls in response to continuous lesser pressure." It has been shown, for instance, by Fischer and Schmieden that in dogs a section of vein transplanted into an artery takes on an arterial structure, at least as regards the circular musculature, which doubles in thickness.

"(5) The power to regulate the normal[487] length of the arteries and veins, in adaptation to the growth of the surrounding tissues, in such a way that the stretching action of the blood-stream brings the vessel to its proper functional length.

"(6) The power to form, in response to slight increases in longitudinal tension, new structural parts which take their place alongside the existing longitudinal fibres.

"(7) The power to regulate the width of the circular musculature according to the degree of food-consumption by the tissues, in response to nerve impulses initiated in these tissues.

"(8) The power possessed by the circular musculature of responding to such continuous functional widening, by the formation of new structural parts in the circular musculature, and so of widening the vessel permanently or by this new formation of muscular fibres thickening the circular musculature.

"(9) The faculty of being stimulated by increased blood-pressure to produce the same structural changes as mentioned in par. 8, though here the response is otherwise conditioned" (pp. 126-7, 1910).

It is by virtue of the tissue-properties detailed above that the complex functional adaptations of the blood-vessels come about.

The development of the vascular system is no mere automatic and mechanical production of form, apart from and independent of functioning; it implies a living and co-ordinated activity of the tissues and organs concerned, a power of active response to foreseen and unforeseen contingencies. Form is then not something fixed and congealed—it is the ever-changing manifestation of functional activity. "Since most of the structure and form of the blood-vessels arises in direct adaptation to function, the vessels of adult men and animals are no fixed structures, which, once formed, retain their form and structural build unchanged throughout life; on the contrary, they require even for their continued existence the stimulus of functional activity.... The fully formed blood-vessels are no static structures, such as they appear to be according to the teaching of normal histology, and such as they have long been taken to be. Observation and description of normal development never shows us anything but the visible side of organic happenings, the products of activity, and leaves us ignorant of the real processes of form-development and form-conservation, and of their causes" (p. 125, 1910).

The real thing in organisation is not form but activity. It is in this return to the Cuvierian or functional attitude to the problems of form that we hold Roux's greatest service to biology to consist. The attitude, however, seems to smack of vitalism, and Roux, as we have seen, is no vitalist. He holds that the marvellous and apparently purposive tissue-qualities which underlie all processes of functional adaptation have arisen "naturally," in the course of evolution, by the action of natural selection upon the various properties, useful and useless, which appeared fortuitously in the primary living organisms. He is, moreover, deeply imbued with the materialistic philosophy of his youth, and it is indeed one of the chief characteristics of his system that he states the fundamental properties or qualities of life in terms of metabolism. A vital quality is for Roux a special process or mode of assimilation. The faculty of "morphological assimilation" whereby form is imposed upon formless chemical processes is the ultimate term of Roux's analysis—"the most general, most essential, and most characteristic formative activity of life" (p. 631, 1902).

We have now to consider very briefly the early results achieved by Roux's fellow-workers in the field of causal morphology. As D. Barfurth points out,[488] the years 1880-90 saw a general awakening of interest in experimental morphology, and it is hard to say whether Roux's work was cause or consequence. "There fall into this period," writes Barfurth, "the experimental investigations by Born and Pflueger on the sexual difference in frogs (1881), by Pflueger on the parthenogenetic segmentation of Amphibian ova, on crossing among the Amphibia, and on other important subjects (1882). In the following year (1883) appeared two papers of fundamental importance, by E. Pflueger and W. Roux: Pflueger publishing his researches on 'the influence of gravity on cell-division,' Roux his experimental investigations on 'the time of the determination of the chief planes in the frog-embryo.'... In the same year appeared A. Rauber's experimental studies 'on the influence of temperature, atmospheric pressure, and various substances on the development of animal ova,' which have brought many similar works in their train. The following year (1884) saw a lively controversy on Pflueger's gravity-experiments with animal eggs, in which took part Pflueger, Born, Roux, O. Hertwig and others, and in this year appeared work by Roux dealing with the experimental study of development, and in particular giving the results of the first definitely localised pricking-experiments on the frog's egg (in the Schles. Gesell. f. vaterl. Kultur, 15th Feb. 1884), also the important researches of M. Nussbaum and Gruber (followed up later by Verworn, Hofer and Balbiani) on Protozoa, and other experimental work" (pp. xi.-xii.).

In 1888 appeared a famous paper by W. Roux,[489] in which he described how he had succeeded in killing by means of a hot needle one of the two first blastomeres of the frog's egg, and how a half-embryo had developed from the uninjured cell. Some years before[490] he had enunciated, at about the same time as Weismann, the view that development was brought about by a qualitative division of the germ-plasm contained in the nucleus, and that the complicated process of karyokinetic or mitotic division of the nucleus was essentially adapted to this end. He conceived that development proceeded by a mosaic-like distribution of potencies to the segmentation-cells, that, for instance, the first segmentation furrow separated off the material and potencies for the right half of the embryo from those for the left half. He had tried to show experimentally that the first furrow in the frog's egg coincided with the sagittal plane of the embryo,[491] and his later success in obtaining a half-embryo from one of the first two blastomeres seemed to establish the "mosaic theory" conclusively.

Roux's needle-experiment aroused much interest, especially as Weismann's theory of heredity was then being keenly discussed. Chabry had published in 1887 some interesting results on the Ascidian egg,[492] which strongly supported the Roux-Weismann theory. Considerable astonishment was therefore caused by Driesch's announcement in 1891[493] that he had obtained complete larvae from single blastomeres of the sea-urchin's egg isolated at the two-celled stage. He followed this up in the next year[493] by showing that whole embryos could be produced from one or more blastomeres isolated at the four-cell stage. Similar or even more striking results were obtained by E. B. Wilson on Amphioxus,[494] and Zoja on medusae.[495] Driesch succeeded also in disturbing the normal course and order of segmentation by compressing the eggs of the sea-urchin between glass plates, and yet obtained normal embryos. Similar pressure-experiments were carried out on the frog by O. Hertwig,[496] and on Nereis by E. B. Wilson,[497] with analogous results.

In 1895 O. Schultze[498] showed that if the frog's egg is held between two plates and inverted at the two-celled stage there are formed two embryos instead of one. In the same year T. H. Morgan[499] repeated Roux's fundamental experiment of destroying one of the two blastomeres, but inverted the egg immediately after the operation—a whole embryo of half size resulted. A year or two later Herlitzka[500] found that if the first two blastomeres of the newt's egg were separated by constriction, two normal embryos of rather more than half normal size were formed.

The main result of the first few years' work on the development of isolated blastomeres was to show that the mosaic theory was not strictly true, and that the hypothesis of a qualitative division of the nucleus was on the whole negatived by the facts.

Evidence soon accumulated that the cytoplasm of the egg stood for much in the differentiation of the embryo. A number of years previously Chun had made the discovery that single blastomeres of the Ctenophore egg, isolated at the two-celled stage, gave half-embryos. This was in the main confirmed by Driesch and Morgan in 1896,[501] and they made the further interesting discovery that the same defective larvae could be obtained by removing from the unsegmented egg a large amount of cytoplasm. Conclusive proof of the importance of the cytoplasm was obtained soon after by Crampton,[502] who removed the anucleate "yolk-lobe" from the egg of the mollusc Ilyanassa at the two-celled stage, and obtained larvae which lacked a mesoblast. This result was brilliantly confirmed and extended some years later by E. B. Wilson,[503] working on the egg of Dentalium. He found that if the similar anucleate "polar lobe" of this form is removed at the two-celled stage, deficient larvae are formed, in which the post-trochal region and the apical organ are absent. He further showed that in the unsegmented but mature egg prelocalised cytoplasmic regions can be distinguished, which later become separated from one another through the segmentation of the egg. The segmentation-cells into which these cytoplasmic substances are thus segregated show a marked specificity of development, giving rise, even when isolated, to definite organs of the embryo. Wilson concluded that the cytoplasm of the egg contains a number of specific organ-forming stuffs, which have a definite topographical arrangement in the egg. Development is thus due in part to a qualitative division not of the nucleus but of the cytoplasm. Corroborative evidence of the existence of cytoplasmic organ-forming stuffs has been supplied for several other species, e.g., Patella (Wilson), Cynthia (Conklin), Cerebratulus (Zeleny), and Echinus (Boveri).

It is interesting to recall that so long ago as 1874 W. His[504] put forward the theory that there exist in the blastoderm and even in the egg prelocalised areas, which contain the formative material for each organ of the embryo, and from which the embryo is developed by a simple process of unequal growth.

The experimental study of form was prosecuted in many other directions besides that of experimental embryology. The study of regeneration and of regulatory processes attracted many workers, among whom may be mentioned T. H. Morgan, C. M. Child, and H. Driesch. In an interesting series of papers C. Herbst applied the principles of the physiology of stimulus to the interpretation of development.[505] The formative power of function was studied in Germany by Roux and his pupils, Fuld, O. Levy, Schepelmann and others, particularly by E. Babak. In France, F. Houssay inaugurated[506] an important series of memoirs by himself and his pupils on "dynamical morphology," the most important memoir being his own valuable discussion of the functional significance of form in fishes.[507] The principles of his dynamical morphology were first laid down in his book La Forme et la Vie (1900).

The famous experiments of Loeb, Delage and others on artificial parthenogenesis may also be mentioned, though their connection with morphology is somewhat remote.

The period was characterised also by the lively discussion of first principles, in which Driesch took a leading part. Materialistic methods of interpretation were upheld by perhaps the majority of biologists, but vitalism found powerful support.

[464] See Carus's remark, referred to on p. 194, above.

[465] Roux, Die Entwicklungsmechanik, p. 26, Leipzig, 1905.

[466] T. H. Morgan, Regeneration, p. 1, New York and London, 1901.

[467] Recherches sur la production artificielle des Monstruosites, Paris, 1877, and many later papers.

[468] Unsere Koerperform und das physiologische Problem ihrer Entstehung, Leipzig, 1874.

[469] J. W. Jenkinson, Experimental Embryology, p. 3, Oxford, 1909.

[470] "Ueber die Verzweigungen der Blutgefaesse des Menschen," Jen. Zeit., xii., 1878.

[471] "Ueber die Bedeutung der Ablenkung des Arterienstammes bei der Astabgabe," Jen. Zeit., xiii., 1879.

[472] "Beitraege zur Morphologie der funktionellen Anpassung. I. Struktur eines hochdifferenzierten bindgewebigen Organes (der Schwanzflosse des Delphin)," Arch. Anat. Physiol. (Anat. Abt.) for 1883. II. "Ueber die Selbstregulation der 'morphologischen' Laenge der Skeletmuskeln des Menschen," Jen. Zeit., xvi., 1883. III. "Beschreibung ... einer Kniegelenkeknochenankylose," Arch. Anat. Physiol. (Anat. Abt.) for 1885.

[473] In 1869 and 1877 respectively (Roux, p. 53, 1905).

[474] Ueber die Zeit. der Bestimmung der Hauptrichtungen des Froschembryo, Leipzig, 1883.

[475] "Ueber den Einfluss der Schwerkraft auf die Teilung der Zellen," Pflueger's Archiv, xxxi., 1883. Also subsequent papers in same journal.

[476] For an account of the classical experiments on the frog's egg, see T. H. Morgan, The Development of the Frog's Egg, New York, 1897.

[477] In a series of "Beitraege zur Entwicklungsmechanik des Embryo," published in various journals from 1884 to 1891, all dealing with the frog's egg. Also in many papers in the Archiv f. Entw. mech., from 1895 onwards.

[478] Die Entwicklungsmechanik der Organismen, eine anatomische Wissenschaft der Zukunft, Wien, 1890.

[479] The first volume contains the important Einleitung or general Introduction.

[480] Gesammelte Abhandlungen ueber Entwicklungsmechanik der Organismen, 2 vols., Leipzig, 1895.

[481] "Fuer unser Programm und seine Verwirklichung," A.E.M., v., pp. 1-80 and 219-342, 1897. "Ueber die Selbstregulation der Lebewesen," A.E.M., xiii., pp. 610-5, 1902. "Die Entwicklungsmechanik, ein neuer Zweig der biologischen Wissenschaft," Heft I. of the Vortraege u. Aufsaetze ueber Entwicklungsmechanik der Organismen, Leipzig, 1905. Oppel and Roux, "Ueber die gestaltliche Anpassung der Blutgefaesse," Heft x., of the Vortraege u. Aufsaetze, Leipzig, 1910.

[482] "Ueber d. funkt. Anpassung des Muskelmagens der Gans," A.E.M., xxi., pp. 461-99, 1906.

[483] The exact quantitative formulation of a Wirkungsweise constitutes a law. The word itself is perhaps most conveniently rendered as "causal process."

[484] M. Fuerbringer, perhaps under the influence of Roux, emphasised the importance, from a morphological point of view, of studying post-embryonic (functional) development, Unters. z. Morph. u. Syst. der Voegel, ii., Amsterdam, p. 925, 1888.

[485] See, for the development of this idea, Oppel, in Roux-Oppel, 1910.

[486] Cf. the controversy between Herbert Spencer and Weismann on the subject of "coadaptation" in the Contemporary Review for 1893 and 1894. See also Weismann's paper in Darwin and Modern Science, Cambridge, 1909.

[487] That is, the length they take up when separated from the body.

[488] "Wilhelm Roux zum 60. Geburtstage," Arch. f. Entw.-Mech., xxx. Festschrift fuer Prof. Roux, Pt. i, 1910.

[489] Virchow's Archiv, cxiv., 1888. First announced in Sept. 1887.

[490] Ueber die Bedeutung der Kernteilungsfiguren, Leipzig, 1883.

[491] Bresl. aertz. Zeitschr., 1885.

[492] Journ. de l'Anat. et de la Physiologie, xxiii., 1887.

[493] Zeits. f. wiss. Zool., liii., 1891 and 1892.

[494] Journ. Morph., viii., 1893.

[495] Arch. f. Ent.-Mech., i., 1895; ii., 1896.

[496] Arch. f. mikr. Anat., xliii., 1893.

[497] Arch. f. Ent.-Mech., iii., 1896.

[498] Arch. f. Ent.-Mech., i., 1895.

[499] Anat. Anz., x., 1895.

[500] Arch. f. Ent.-Mech., iv. 1897.

[501] Arch. f. Ent.-Mech., ii., 1896.

[502] Arch. f. Ent.-Mech., iii., 1896.

[503] Journ. exper. Zool., i., 1904.

[504] Unsere Koerperform, p. 19, Leipzig, 1874.

[505] Biolog. Centrlbl., xiv., 1894, xv., 1895. Formative Reize in der thierischen Ontogenese, Leipzig, 1901.

[506] "La Morphologie dynamique," No. i. of the Collection de Morphologie dynamique, Paris, 1911.

[507] "Forme, Puissance et Stabilite des Poissons," No. iv. of the Collection, Paris, 1912.



CHAPTER XIX

SAMUEL BUTLER AND THE MEMORY THEORIES OF HEREDITY

We have laid stress upon the distinction established by Roux between the two stages of development—the automatic and the functional—because of the light which it seems to throw upon the phylogenetic relation of form to function. We have pointed out, too, the paramount role that function plays in Roux's theories of development and heredity, and we have brought out the close kinship existing between his theory and that of Lamarck. For Roux, as for Lamarck, the function creates the organ, and it is only after long generations that the organ appears before the function.

It so happened that just about the time when Roux's papers were beginning to appear a brilliant attempt was made by Samuel Butler to revive and complete the Lamarckian doctrine.

A man of singular freshness and openness of mind, combining in an extraordinary degree extreme intellectual subtlety with a childlike simplicity of outlook, Butler was one of the most fascinating figures of the 19th century. He was not a professional biologist, and much of his biological work is, for that reason, imperfect. But he brought to bear upon the central problems of biology an unbiassed and powerful intelligence, and his attitude to these problems, just because it is that of a cultivated layman, is singularly illuminating.

He was not well acquainted with biological literature; he seems to have hit upon the main ideas of his theory of life and habit in complete independence of Lamarck, and only later to have become aware that Lamarck had in a measure forestalled him. He puts this very beautifully in the following passage from his chief biological work Life and Habit (1877[508]):—"I admit that when I began to write upon my subject I did not seriously believe in it. I saw, as it were, a pebble upon the ground, with a sheen that pleased me; taking it up, I turned it over and over for my amusement, and found it always grow brighter and brighter the more I examined it. At length I became fascinated, and gave loose rein to self-illusion. The aspect of the world changed; the trifle which I had picked up idly had proved to be a talisman of inestimable value, and had opened a door through which I caught glimpses of a strange and interesting transformation. Then came one who told me that the stone was not mine, but that it had been dropped by Lamarck, to whom it belonged rightfully, but who had lost it; whereon I said I cared not who was the owner, if only I might use it and enjoy it. Now, therefore, having polished it with what art and care one who is no jeweller could bestow upon it, I return it, as best I may, to its possessor" (p. 306). In one of his later works, however, Butler made up for his first neglect of his predecessors by giving what is undeniably the best account in English literature of the work of Buffon, Lamarck, and Erasmus Darwin—in his Evolution, Old and New (1879). Many of his facts he took from Charles Darwin, whose theory of natural selection he bitterly opposed, in the two books just mentioned and in Unconscious Memory (1880) and Luck or Cunning (1887).

Butler's main thesis is that living things are active, intelligent agents, personally continuous with all their ancestors, possessing an intense but unconscious memory of all that their ancestors did and suffered, and moving through habit from the spontaneity of striving to the automatism of remembrance.

The primary cause of all variation in structure is the active response of the organism to needs experienced by it, and the indispensable link between the outer world and the creature itself is that same "sense of need" upon which Lamarck insisted. "According to Lamarck, genera and species have been evolved, in the main, by exactly the same process as that by which human inventions and civilisations are now progressing; and this involves that intelligence, ingenuity, heroism, and all the elements of romance, should have had the main share in the development of every herb and living creature around us" (Life and Habit, p. 253). Variations are indubitably the raw material of evolution—"The question is as to the origin and character of these variations. We say they mainly originate in a creature through a sense of its needs, and vary through the varying surroundings which will cause those needs to vary, and through the opening-up of new desires in many creatures, as the consequence of the gratification of old ones; they depend greatly on differences of individual capacity and temperament; they are communicated, and in the course of time transmitted, as what we call hereditary habits or structures, though these are only, in truth, intense and epitomised memories of how certain creatures liked to deal with protoplasm" (p. 267).

Butler's theory then is essentially a bold and enlightened Lamarckism, completed and rounded off by the conception that heredity too is a psychological process, of the same nature as memory.

In seeking to establish a close analogy between memory and heredity Butler starts out from the fact of common experience, that actions which on their first performance require the conscious exercise of will and intelligence, and are then carried out with difficulty and hesitation, gradually through long-continued practice come to be performed easily and automatically, without the conscious exercise of intelligence or will.

He tries to show that this is a general law—that knowledge and will become intense and perfect only when through long-continued exercise they become automatic and unconscious—and he applies this conception to the elucidation of development.

Developmental processes, especially the early ones (of Roux's first stage) are automatic and unconscious, and yet imply the possession by the embryo of a wonderfully perfect knowledge of the processes to be gone through, and an assured power of will and judgment. Is it conceivable, says Butler, that the embryo can do all these things without knowing how to do them, and without having done them before? "Shall we say ... that a baby of a day old sucks (which involves the whole principle of the pump, and hence a profound practical knowledge of the laws of pneumatics and hydrostatics), digests, oxygenises its blood (millions of years before Sir Humphrey Davy discovered oxygen), sees and hears—all most difficult and complicated operations, involving a knowledge of the facts concerning optics and acoustics, compared with which the discoveries of Newton sink into utter insignificance? Shall we say that a baby can do all these things at once, doing them so well and so regularly, without being even able to direct its attention to them, and without mistake, and at the same time not know how to do them, and never have done them before?" (p. 54). Assuredly not.

The only possible explanation is that the embryo's ancestors have done these things so often, throughout so many millions of generations, that the embryo's knowledge of how to do them has become unconscious and automatic by reason of this age-long practice. This implies that there is in a very real sense actual personal continuity between the embryo and all its ancestors, so that their experiences are his, their memory also his. "We must suppose the continuity of life and sameness between living beings, whether plants or animals, to be far closer than we have hitherto believed; so that the experience of one person is not enjoyed by his successor, so much as that the successor is bona fide but a part of the life of his progenitor, imbued with all his memories, profiting by all his experiences—which are, in fact, his own—and only unconscious of the extent of his own memories and experiences owing to their vastness and already infinite repetitions" (p. 50). It is very suggestive in this connection, he continues—"I. That we are most conscious of, and have most control over, such habits as speech, the upright position, the arts and sciences, which are acquisitions peculiar to the human race, always acquired after birth, and not common to ourselves and any ancestor who had not become entirely human.

"II. That we are less conscious of, and have less control over, eating and drinking, swallowing, breathing, seeing and hearing, which were acquisitions of our prehuman ancestry, and for which we had provided ourselves with all the necessary apparatus before we saw light, but which are, geologically speaking, recent, or comparatively recent.

"III. That we are most unconscious of, and have least control over, our digestion and circulation, which belonged even to our invertebrate ancestry, and which are habits, geologically speaking, of extreme antiquity.... Does it not seem as though the older and more confirmed the habit, the more unquestioning the act of volition, till, in the case of the oldest habits, the practice of succeeding existences has so formulated the procedure, that, on being once committed to such and such a line beyond a certain point, the subsequent course is so clear as to be open to no further doubt, to admit of no alternative, till the very power of questioning is gone, and even the consciousness of volition" (pp. 51-2).

The hypothesis then, that heredity and development are due to unconscious memory, finds much to support it—"the self-development of each new life in succeeding generations—the various stages through which it passes (as it would appear, at first sight, without rhyme or reason), the manner in which it prepares structures of the most surpassing intricacy and delicacy, for which it has no use at the time when it prepares them, and the many elaborate instincts which it exhibits immediately on, and indeed before, birth—all point in the direction of habit and memory, as the only causes which could produce them" (p. 125). The hypothesis explains, for instance, the fact of recapitulation:—"Why should the embryo of any animal go through so many stages—embryological allusions to forefathers of a widely different type? And why, again, should the germs of the same kind of creature always go through the same stages? If the germ of any animal now living is, in its simplest state, but part of the personal identity of one of the original germs of all life whatsoever, and hence, if any now living organism must be considered without quibble as being itself millions of years old, and as imbued with an intense though unconscious memory of all that it has done sufficiently often to have made a permanent impression; if this be so, we can answer the above questions perfectly well. The creature goes through so many intermediate stages between its earliest state as life at all, and its latest development, for the simplest of all reasons, namely, because this is the road by which it has always hitherto travelled to its present differentiation; this is the road it knows, and into every turn and up or down of which it has been guided by the force of circumstances and the balance of considerations" (pp. 125-6).

The hypothesis explains also the way in which the orderly succession of stages in embryogeny is brought about, for we can readily understand that the embryo will not remember any stage until it has passed through the stage immediately preceding it. "Each step of normal development will lead the impregnated ovum up to, and remind it of, its next ordinary course of action, in the same way as we, when we recite a well-known passage, are led up to each successive sentence by the sentence which has immediately preceded it.... Though the ovum immediately after impregnation is instinct with all the memories of both parents, not one of these memories can normally become active till both the ovum itself and its surroundings are sufficiently like what they respectively were, when the occurrence now to be remembered last took place. The memory will then immediately return, and the creature will do as it did on the last occasion that it was in like case as now. This ensures that similarity of order shall be preserved in all the stages of development in successive generations" (pp. 297-8).

Abnormal conditions of development will cause the embryo to pause and hesitate, as if at a loss what to do, having no ancestral experience to guide it. Abnormalities of development represent the embryo's attempt to make the best of an unexpected situation. Or, as Butler puts it, "When ... events are happening to it which, if it has the kind of memory we are attributing to it, would baffle that memory, or which have rarely or never been included in the category of its recollections, it acts precisely as a creature acts when its recollection is disturbed, or when it is required to do something which it has never done before" (p. 132). "It is certainly noteworthy that the embryo is never at a loss, unless something happens to it which has not usually happened to its forefathers, and which in the nature of things it cannot remember" (p. 132).

Butler's teleological conception of organic evolution was of course completely antagonistic to the naturalistic conceptions current in his time. In one of his later books he repeats Paley's arguments in favour of design, and to the question, "Where, then, is your designer of beasts and birds, of fishes, and of plants?" he replies: "Our answer is simple enough; it is that we can and do point to a living tangible person with flesh, blood, eyes, nose, ears, organs, senses, dimensions, who did of his own cunning, after infinite proof of every kind of hazard and experiment, scheme out and fashion each organ of the human body. This is the person whom we claim as the designer and artificer of that body, and he is the one of all others the best fitted for the task by his antecedents, and his practical knowledge of the requirements of the case—for he is man himself. Not man, the individual of any given generation, but man in the entirety of his existence from the dawn of life onwards to the present moment" (Evolution, Old and New, p. 30, 1879).

Butler's theory of life and habit remained only a sketch, and he was perhaps not fully aware of its philosophical implications. Since Butler's time, a new complexion has been put upon biological philosophy by the profound speculations of Bergson.

But it is not impossible that the future development of biological thought will follow some such lines as those which he tentatively laid down.

Butler was not the first to suggest that there is a close connection between heredity and memory—it is a thought likely to occur to any unprejudiced thinker. The first enunciation of it which attracted general attention was that contained in Hering's famous lecture "On Memory as a general Function of organised Matter."[509] Butler was not aware of Hering's work when he published his Life and Habit, but in Unconscious Memory (1880) he gave full credit to Hering as the first discoverer, and supplied an admirable translation of Hering's lecture. As far as the assimilation of heredity to memory is concerned Hering and Butler have much in common, but Hering did not share Butler's Lamarckian and vitalistic views, preferring to hold fast, for the practical purposes of physiology at all events, to the general accepted theory of the parallelism between psychical and physical processes. He was inclined to regard memory in the ordinary sense as a function of the brain, and memory in general as a function of all organised matter. Speaking of the psychical life, he says, "Thus the cause which produces the unity of all single phenomena of consciousness must be looked for in unconscious life. As we know nothing of this except what we learn from our investigations of matter, and since in a purely empirical consideration, matter and the unconscious must be regarded as identical, the physiologist may justly define memory in a wider sense to be a faculty of the brain, the results of which to a great extent belong to both consciousness and unconsciousness."[510] Hering's views were supported by Haeckel.[511]

In 1893 an American, H. F. Orr,[512] tried to work out a theory of development and heredity based upon the fundamental idea "that the property which is the basis of bodily development in organisms is the same property which we recognise as the basis of psychic activity and psychic development." He tried also to explain the recapitulation of phylogeny by ontogeny as due to habit.

The neo-Lamarckian school of American palaeontologists were also in sympathy with the memory idea, and this was expressed most clearly perhaps by Cope.[513]

In 1904 appeared the work on this subject which has attracted the most attention—R. Semon's Die Mneme.[514] This was an elaborate treatment of the question from the materialistic point of view, the main assumption of Semon's theory being that the action of a stimulus upon the organism leaves a more or less permanent material trace or "engramm," of such a nature as to modify the subsequent action of the organism.

Applied to the explanation of heredity and development, Semon's theory comes to very much the same as Weismann's, with engramms substituted for determinants, but it has the great advantage of allowing for the transmission of acquired characters. The application of the concept of stimulus is valuable and suggestive, but it seems to us that the memory theory of heredity can be properly utilised only by adopting a frankly Lamarckian and vitalistic standpoint, and this standpoint Semon expressly combats. As Ward[515] points out in his illuminating lecture on heredity and memory—"Records or memoranda alone are not memory, for they presuppose it. They may consist of physical traces, but memory, even when called 'unconscious,' suggests mind; for, as we have seen, the automatic character implied by this term 'unconscious' presupposes foregone experience.... The mnemic theory then, if it is to be worth anything, seems to me clearly to require not merely physical records or 'engrams,' but living experience or tradition. The mnemic theory will work for those who can accept a monadistic or pampsychist interpretation of the beings that make up the world, who believe with Spinoza and Leibniz that 'all individual things are animated albeit in divers degree'" (pp. 55-6).

Perhaps the best and most ingenious treatment of memory and heredity from a physical standpoint is that offered by E. Rignano in his book, Sur la transmissibilite des caracteres acquis.[516] Rignano seeks to construct a physico-chemical "model" which will explain both heredity and memory.

His system, which is based more firmly upon the facts of experimental embryology than Semon's, postulates the existence of "specific nervous accumulators." The essential hypothesis set up is that every functional stimulus is transformed into specific vital energy, and deposits in the nucleus of the cell a specific substance which is capable of discharging, in an inverse direction, the nervous current which has formed it, as soon as the dynamical equilibrium of the organism is restored to the state in which it was when the original stimulus acted upon it. These specific nuclear substances, different for each cell, are accumulated also in the nuclei of the germinal substance, constituting what Rignano calls the central zone of development. That is to say, each functional adaptation changes slightly the dynamical equilibrium of the organism, and this change in the system of distribution of the nervous currents leads to the deposit in the central zone of development of a new specific substance. In the development of the next individual this new specific element enters into activity, and reproduces the nervous current which has formed it, as soon as the organism reaches the same conditions of dynamical equilibrium as those obtaining when the stimulus acted on the parent.

Development can thus be regarded as consisting of a number of stages, at each of which new specific elements enter automatically into play and lead the embryo from that stage to the stage succeeding. The germinal substance on this theory of Rignano's is to be regarded as being composed of a large number of specific elements, originally formed as a result of each new functional adaptation, but now forming part of the hereditary equipment.

The theory represents an advance upon the more static conceptions of Semon. It owes much to Roux's influence.

In this country, the mnemic theories have been championed particularly by M. Hartog[517] and Sir Francis Darwin.[518]

[508] The quotations are taken from the 1910 reprint, London, Fifield.

[509] Ueber das Gedaechtnis als eine allgemeine Funktion der organisierten Materie, Wien, 1870.

[510] Eng. trans, in E. Hering, Memory, p. 9, Chicago and London, 1913.

[511] Die Perigenesis der Plastidule, Jena, 1875.

[512] A Theory of Development and Heredity, New York, 1893.

[513] The Primary Factors of Organic Evolution, Chicago, 1896.

[514] Die Mneme als erhaltendes Prinzip im Wechsel des organischen Geschehens, Leipzig, 1904; 2nd ed., 1908.

[515] Heredity and Memory, Cambridge, 1913.

[516] Paris, 1906. Also in Italian and German. Eng. trans. by B. C. ,H. Harvey, Chicago, 1911.

[517] See Problems of Life and Reproduction, London, 1913.

[518] Presidential Address to the British Association, 1908.



CHAPTER XX

THE CLASSICAL TRADITION IN MODERN MORPHOLOGY

To write a history of contemporary movements from a purely objective standpoint is well recognised to be an impossible task. It is difficult for those in the stream to see where the current is carrying them: the tendencies of the present will only become clear some twenty years in the future.

I propose, therefore, in this concluding chapter to deal only with certain characteristics of modern work on the problems of form which seem to me to be derived directly from the older classical tradition of Cuvier and von Baer.

The present time is essentially one of transition. Complete uncertainty reigns as to the main principles of biology. Many of us think that the materialistic and simplicist method has proved a complete failure, and that the time has come to strike out on entirely different lines. Just in what direction the new biology will grow out is hard to see at present, so many divergent beginnings have been made—the materialistic vitalism of Driesch, the profound intuitionalism of Bergson, the psychological biology of Delpino, France, Pauly, A. Wagner and W. Mackenzie. But if any of these are destined to give the future direction to biology, they will in a measure only be bringing biology back to its pre-materialistic tradition, the tradition of Aristotle, Cuvier, von Baer and J. Mueller. It may well be that the intransigent materialism of the 19th century is merely an episode, an aberration rather, in the history of biology—an aberration brought about by the over-rapid development of a materialistic and luxurious civilisation, in which man's material means have outrun his mental and moral growth.

Two movements seem significant in the morphology of the last decade or so of the 19th century—first, the experimental study of form, and second, the criticism of the concepts or prejudices of evolutionary morphology.

The period was characterised also by the great interest taken in cytology, following upon the pioneer work of Hertwig, van Beneden and others on the behaviour of the nuclei in fertilisation and maturation.[519] This line of work gained added importance in connection with contemporary research and speculation on the nature of hereditary transmission, and it has in quite recent years received an additional stimulus from the re-discovery of Mendelian inheritance. Its importance, however, seems to lie rather in its possible relation to the problems of heredity than in any meaning it may have for the problems of form. More significant is the revolt against the cell-theory started by Sedgwick[520] and Whitman,[521] on the ground that the organism is something more than an aggregation of discrete, self-centred cells.

The experimental work on the causes of the production and restoration of form infused new life into morphology. It opened men's eyes to the fact that the developing organism is very much a living, active, responsive thing, quite capable of relinquishing at need the beaten track of normal development which its ancestors have followed for countless generations, in order to meet emergencies with an immediate and purposive reaction. It was cases of this kind, cases of active regulation in development and regeneration, that led men like G. Wolff and H. Driesch to cast off the bonds of dogmatic Darwinism and declare boldly for vitalism and teleology.

There was the famous case of the regeneration of the lens in Amphibia from the edge of the iris—an entirely novel mode of origin, not occurring in ontogeny. The fact seems to have been discovered first by Colucci in 1891, and independently by G. Wolff in 1895.[522] The experiment was later repeated and confirmed by Fischel and other workers. Wolff drew from this and other facts the conclusion that the organism possesses a faculty of "primary purposiveness" which cannot have arisen through natural selection.[523] And, as is well known, Driesch derived one of his most powerful arguments in favour of vitalism from the extraordinary regenerative processes shown by Tubularia and Clavellina in the course of which the organism actually demolishes and rebuilds a part or the whole of its structure. But under the influence of physiologists like Loeb many workers held fast to materialistic methods and conceptions.

The great variety of regulative response of which the organism showed itself capable made it very difficult for the morphologist to uphold the generalisations which he had drawn from the facts of normal undisturbed development. The germ-layer theory was found inadequate to the new facts, and many reverted to the older criterion of homology based on destiny rather than origin. The trend of opinion was to reject the ontogenetic criterion of homology, and to refuse any morphological or phylogenetic value to the germ-layers.[524]

The biogenetic law came more and more into disfavour, as the developing organism more and more showed itself to be capable of throwing off the dead-weight of the past, and working out its own salvation upon original and individual lines.[525] A. Giard in particular called attention to a remarkable group of facts which went to show that embryos or larvae of the same or closely allied species might develop in most dissimilar ways according to the conditions in which they found themselves.[526] His classical case of "poecilogeny" was that of the shrimp Palaemonetes varians, the fresh-water form of which develops in an entirely different way from the salt-water form.

Experimental workers indeed were inclined to rule the law out of account, to disregard completely the historical element in development, and this was perhaps the chief weakness of the neo-vitalist systems which took their origin in this experimental work.

From the side also of descriptive morphology the biogenetic law underwent a critical revision. It was studied as a fact of embryology and without phylogenetic bias by men like Oppel, Keibel, Mehnert, O. Hertwig and Vialleton,[527] and they arrived at a critical estimate of it very similar to that of von Baer.

Theoretical objections to the biogenetic law had been raised from time to time by many embryologists, but the positive testing of it by the comparison of embryos in respect of the degree of development of their different organs starts with Oppel's work of 1891.[528] He studied a large number of embryos of different species at different stages of their development, and determined the relative time of appearance of the principal organs and their relative size. His results are summarised in tabular form and have reference to all the more important organs. He was led to ascribe a certain validity to the biogenetic law, but he drew particular attention to the very considerable anomalies in the time of appearance which are shown by many organs, anomalies which had been classed by Haeckel under the name of heterochronies.