To take first the loss of pigmentation from the lower side. I have shown experimentally that exposure of the lower sides of Flounders to light reflected upwards from below causes development of pigment on the lower side. At the same time the experiments proved that the loss of pigment in the fish in the natural state and the development of it under exposure to light were not merely direct results of the presence or absence of light in the individual, for in some cases the young fish were placed in the apparatus before the pigment had entirely disappeared from the lower side, and the metamorphosis went on, the lower side becoming quite white, and the pigment only developed gradually after long exposure to the light. In the principal experiment four specimens were placed in the apparatus on September 17, 1890, when about six months old and 7 to 9 cm. in length. One of these died on July 1, 1891, and had no pigment on the lower side. The other three all developed pigment on that side. In one it was first noticed in April 1891, and in the following November the fish was 22 cm. long and had pigmentation over the greater part of the lower side (Plate III.). Microscopically examined, the pigmentation was found to consist of black and orange chromatophores exactly similar to those of the upper side. Some hundreds of young Flounders were reared at the same time under ordinary conditions and none of them developed pigment.

It is clear, therefore, that exposure of the lower side to light and reduction of the amount of light falling on the upper side (for the tops of the aquaria used were covered with opaque material) does not cause the two sides to behave in the same way in respect of pigment, as they would if the normal condition of the fish was merely due to the difference in the exposure to light of the two sides in the individual life. There is a very strong congenital or hereditary tendency to the disappearance of pigment from the lower side, and this is only overcome after long exposure to the light. On the other hand, if the disappearance of the pigment were due to a mutation, were gametogenic and entirely independent of external conditions, there would be no development of pigment after the longest exposure. To prove that an inherited character is an acquired character is quite as good evidence as to show that an acquired character is inherited. The latter kind of evidence is very difficult to get, for the effect of conditions in a single lifetime is but slight, and is not likely to show a perceptible inherited effect. The theory that adaptations are due to the heredity of the effects of stimulation assumes that the same stimulus has been acting for many generations.



It is necessary, however, to consider how far the conclusions drawn from these experiments are contradicted by the mutations occurring in nature, some of which have already been mentioned. We will consider first ambicolorate specimens. If the absence of pigment from the lower side in normal Flat-fishes is due to the absence of light, how is it that the pigmentation persists on the lower side of ambicolorate specimens, which is no more exposed to light than in normal specimens? The answer is that in the mutants the determinants for pigmentation are united with the determinants for the lower side of the fish. My view is that the differentiation of these determinants for the two sides was due in the course of evolution to the different exposure to light, was of somatic origin, but once the congenital factors or determinants were in existence they were liable to mutation, and thus in the ambicolorate specimens there is a congenital tendency to pigmentation on the lower side, which would only be overcome by exclusion of light for another series of generations.

Mutations also occur in which part or whole of the upper side is white and unpigmented. Several such specimens are mentioned in the memoir by myself and Dr. MacMunn in the Phil. Trans. already cited, one being a Sole which was entirely white on the lower side, and also on the upper, which was pigmented only over the head region from the free edge of the operculum forwards. Since the upper sides in these specimens are fully exposed to light in the natural state and yet remain unpigmented, it would appear impossible to believe that the action of light was the cause of the development of pigment on the lower sides of normal specimens in my experiments. To some it may be so, but in my own opinion the one fact is as certain as the other. I believe the two facts can be reconciled. I had one specimen of Plaice in the living condition which had the middle third of its upper surface white, and the whole of the lower side white as usual. This specimen was kept for 4-1/2 months with its lower surface exposed to light and the upper side shaded. At the end of that period there were numerous small patches of pigment scattered over the lower side principally in the regions of the interspinous bones, above and below the lateral line. In the area of the upper side, which was originally unpigmented, there were also numerous small pigment spots. I believe, therefore, that in this case there were determinants for absence of pigment not only on the lower side but on part of the upper side also, and that so long as light was excluded from the lower side the patch on the upper side remained unpigmented in sympathy. When the congenital tendency of the determinants on the lower side was overcome by the action of light, the white patch on the upper side also began to develop pigment.

Lastly, I may refer again to the specially abnormal Turbot mentioned above. In this case the lower side was over the greater part pigmented and the upper side white, and this would appear to contradict the conclusion just drawn concerning the piebald Plaice. But this Turbot was only 4.4 cm. long, and is the only case known to me where so much of the lower side was pigmented with the upper side almost entirely white. The theory of sympathy or correlation might apply here since the lower side of the head was unpigmented, but from the small size of the specimen and the amount of pigment on the lower side, it seems to me most probable that if the specimen had lived to be adult the upper side would have developed pigment under the action of light and the specimen would have become ambicolorate.

When we compare the results reached by the mutationists with those obtained by the Mendelians we find that they tend to two different conceptions of the relation between the gametes and the organism developed from them. The effect of a change in the determinants of the gametes according to the mutationists is evident in every part of the plant. A factor in Mendelian experiments usually affects only one organ or one part of the organism. The factor for double hallux in fowls, for instance, may coexist with single comb or rose comb. The general impression produced on the mind by study of Mendelian phenomena is that the organism is a mosaic of which every element corresponds to a separate element in the chromosomes. Thus we know that what we call a single factor may cause the whole plumage of a fowl to have the detached barbs, which constitutes the Silky character, but we also know that an animal may be piebald, strongly pigmented in one part and white or unpigmented in another. So we find in these Flat-fish mutations mosaic-like forms which evidently result from mosaic-like factors in the gametes, or in the chromosomes of the gametes.

Experimental evidence concerning the movement of the lower eye to the upper side and of the forward extension of the dorsal fin has not been obtained, though years ago I made some attempts, at the suggestion of Mr. G. J. Romanes, to obtain such evidence with regard to the eye by keeping young Flounders, already partially metamorphosed, in a reversed position. I did not succeed in devising apparatus which would keep the young fish alive in the reversed position for a sufficiently long time. We can only consider, therefore, whether those other changes can reasonably be attributed to the conditions of life. Anatomical investigation shows that the bony interorbital septum composed principally of the frontal bones, which in symmetrical fish passes between the eyes, is still between the eyes in the Flat-fish, but has been bent round through an angle of 90 degrees on the upper side, while in the lower side a new bony connexion has been formed on the outer side of the eye which has moved from the lower side. This connexion is due to a growth from the prefrontal backwards to join a process of the frontal, and is entirely absent in symmetrical fishes. It is along this bony bridge that the dorsal fin extends. The origin of the eye muscles and of the optic nerves is morphologically the same as in symmetrical fishes. On the theory of modification by external stimuli we must naturally attribute the dislocation of the eye of the lower side to the muscular effort of the fish to direct this eye to the dorsal edge, but something may also be due to the pressure of the flat ground on the eye-ball. There is little difficulty in attributing the bending of the interorbitl septum to pressure of the lower eye-ball against it, pressure which is probably due partly if not chiefly to the action of the eye muscles. The formation of the bony bridge outside the dislocated eye is more difficult to explain, as I have never had the opportunity to study the relation of this bridge to the muscles. It is worth mentioning that in the actual development of Turbot and Brill the metamorphosis takes place to a considerable degree while the young fish is pelagic, before the habit of lying on the ground is assumed, but of course this is no evidence that the change was not originally caused by the habit of lying on the ground.

With regard to the extension of the dorsal fin there is no difficulty in discovering a stimulus which would account for it. Symmetrical fishes propel themselves chiefly by the tail; in shuffling over the ground or swimming a little above it. Flat-fishes move by means of undulations of the dorsal and ventral fins. Increased movement produces hypertrophy, and according to the theory here maintained, not merely enlargement of parts existing, but phylogenetic increase in the number of such parts, here fin rays and their muscles. In Flat-fishes the dorsal and ventral fins extend along the whole length of the dorsal and ventral edges: the dorsal from the head, in some cases from a point anterior to the eyes, to the base of the tail, the ventral from the anus, which is pushed very far forward, to the base of the tail, and in some species of Solidae these fins are confluent with the caudal fin.

Formerly it was dogmatically maintained that the effect of an external stimulus on somatic organs or tissues could have no influence on the determinants in the chromosomes of the gametes to which the hereditary characters of the organism were due. As we have tried to show, this dogma is no longer credible in face of the discoveries concerning hormones. The hormone theory supposes that the somatic modifications due to external stimuli—in the case of the Flat-fish the disappearance of pigment from the lower side, the torsion of the orbital region of the skull, and the extension of the dorsal fin—modify the hormones given off by these parts, increasing some and decreasing others, and that these changes in the hormones affect the determinants, whatever they are, in the gametocytes within the body.

Here arises an interesting question—namely, how does the hormone theory explain the phenomenon of metamorphosis any better than the mutation theory? It might be agreed that if the determinants are stimulated or deprived of stimulation, the effect of the change should logically show itself from the beginning of development, and that therefore the process of metamorphosis or indirect development does not support the hormone theory any more than the theory of gametogenic mutations. This objection may be answered in the following way. The reason why the determinants give rise to the original structure first and then change it into the new structure is probably the same as that which causes secondary sexual characters to develop only at the stage of puberty. By the hypothesis the new habits and new stimuli begin to act at some stage after the complete development of the original structure of the body. The differences in the original hormones of the modified parts are therefore acting simultaneously with the hormones, that is, the chemical substances derived from all other parts of the body in its fully developed condition. It is very probable that in the early stages of development the metabolism of the body would be considerably different from that of the adult stage, and the same combination of hormones would not be present. We may suppose, therefore, that the determinants of the zygote have acquired a tendency to produce the increases and decreases of tissue which constitute a certain modification, e.g. the change in the position of the eyes in a Flat-fish, but the stimulus which caused this tendency has always acted when the adult combination of hormones was present. In consequence of this the developed tissues do not undergo the inherited modification until the adult combination is again present. In this way we can form a definite conception of the reason why an adaptive modification is inherited at the same stage in which it was produced, just as the antlers of a stag are only developed when the hormone of the mature testis is present. At the same time it is probable that the age at which the inherited development takes place tends to become earlier in later generations, to occur in fact as soon as the necessary hormone medium is present.

The diagnostic characters, of some of the species of Pleuronectidae have been mentioned in an earlier part of this volume, in order to point out that they have no relation to differences of habit or external conditions. Here it is to be pointed out that there is no evidence that they arise by metamorphosis. The scales, for example, afford distinct and constant diagnostic characters both of species and genera, but their peculiarities have not been found to arise by modification of a primitive form. The rough tubercles of the Flounder, and the scattered thornlike tubercles of the Turbot, develop directly, not by the continuous modification of imbricated scales. There is, however, one scale-character among the Pleuronectidae which appears to stand in direct contradiction to the conclusions drawn by me concerning scales in general. It not only develops by a gradual change, but it is a secondary sexual character developing in the males only at maturity. The character was described by E. W. L. Holt in specimens of the Baltic variety of the Plaice, Pleuronectes platessa, [Footnote: Journ. Mar. Biol. Assn., vol iii. (Plymouth, 1893-95.)] and consists in the spinulation of the posterior edges of the scales, especially on the upper side, in mature males. The same condition, but to a much slighter degree, was afterwards shown by myself to occur constantly in Plaice from the English Channel and North Sea. [Footnote: Ibid., vol. iv. p. 323.] It occurs also in P. glacialis, the representative of the Plaice in more northern seas. I have shown that the spinules develop in the mature males not as a modification of the scale, but as separate calcareous deposits the bases of which afterwards become united to the scale. It would seem that the development of this character is dependent on the hormone from the mature testis, and in order to conform with the arguments used by me in other cases, the spinulation should have some definite function in relation to the habits of the sexes, and this function should involve some kind of external stimulation restricted to the mature male. So far, however, no evidence whatever of such function or such stimulation has been discovered. It is possible that the case differs from other secondary sexual characters as the antlers of stags in one respect, namely, that the Dab (P. limanda), the Sole, and other species of Solea. and several other Pleuronectidae have what are called etenoid scales—that is, scales furnished with spines on the posterior edge—and since the ordinary scales of the Plaice are reduced, the spinulation of scales in the mature male Plaice is not a new character but the retention of a primitive character. Then the question would remain why the scales in the mature female and immature male have degenerated, or rather why the primitive character develops only in the mature stage of the male.

There is one point in which this sexual dimorphism in the Plaice appears to differ from typical cases, and which suggests that the greater spinulation of scales in the males has no function at all in the relations of the sexes, and is therefore not subject to and external stimulation. This point is the remarkable way in which the degree of development of spiny armature differs in different regions and in local races, and seems to correspond to different climatic conditions. Both Plaice and Flounders in the Baltic are much more spiny than in the North Sea, although in the Flounder no sexual difference in this respect has been noted. On the east coast of North America occurs P. glacialis, in which the scales of the male are strongly spinulate and those of the female smooth. On the coast of Alaska females of this species seem to be more spinulate than elsewhere. The Flounder does not occur in the Arctic, but on the west coast of North America occurs a local form called P. stellatus, scarcely distinct as a species, which has a strong development of spiny tubercles all over the upper side. The Flounders of the Mediterranean are much less spinous than those of the North Sea or Channel. The Dab (P. limanda) occurs on the American coast in a local form called Limanda ferruginea, and in the North Pacific there is a rougher form called L. aspera. In these three species therefore, apart from mutations, the northern forms all show a greater development of spines on the scales. Whether this is an effect of colder temperature it is difficult to say. It is possible that the difference is due to external conditions, of which lower temperature of the water is the most obvious, and it may be that these conditions have a greater effect on the male than on the female in the Plaice.

Sexual differences in scales, which have a function in the relations of the sexes, occur in a few other fishes, and these can be attributed with good reason to mechanical stimulation. For example, in the Rajidae among Elasmobranchs the males possess on each 'wing' or pectoral two series of large, recurved, hooked spines. It has been stated, [Footnote: Darwin, Descent of Man (2nd edit., 1885), p. 331.] apparently by Yarrell, that these spines are developed only in the breeding season. It is doubtful if there is any marked breeding season in these fishes, but it is probable that the spines are absent in the immature male, as it is known that in Raia clavata the adult male has sharp pointed teeth, while the young male and the female at all ages have broad flat teeth. It is supposed that the spines and perhaps the sharp teeth are used for holding the female, but it seems equally probable that these structures are really used by the males in fighting with each other. The habits of these marine fish have not been much observed, but there is little reason to doubt that these differences in scales and teeth correspond with differences of mechanical stimulation. This does not at all imply that the scales and teeth themselves have been produced by mechanical stimulation, or that the difference between the dermal denticles of Elasmobranchs and the scales of Teleosteans correspond to differences of stimulation. But the degree of development of a structure whose presence is due to gametic factors may very probably be modified by external stimulation, and the modification may become hereditary. If the views here advocated are true, the two processes mutation and modification must be always acting together and affecting the development not only of the individual but of any organ or structure. Thus the peculiarities of antlers in stags, it seems to me, prove that the mechanical stimulation due to fighting was the cause of the evolution of antlers, that without the habit of fighting in the males antlers would not exist. At the same time each species of the Cervidae has its special characters in the antlers, in shape and branching, and it would be impossible to attribute these to differences in mode of fighting: they are due to mutation.

In connexion with the metamorphosis of Amphibia the case of the Axolotl has always been of very great interest. In the few small lakes near the city of Mexico where it occurs it has never been known to undergo metamorphosis but is aquatic throughout its life and breeds in that condition. Yet in captivity by reducing the quantity of water in which it is placed the young Axolotl can be forced to breathe air, and then it undergoes complete metamorphosis to the abranchiate condition. The same species in other parts of North America normally goes through the metamorphosis, like other species of the Urodela. It is evident, therefore, that the Mexican Axolotls, although they have been perennibranchiate for a great number of generations, have not lost the hereditary tendency to the metamorphosis which changes the larvae of Amblystoma elsewhere into an air-breathing terrestrial animal. This may be regarded as evidence that the conditions of life which prevent the metamorphosis in the Mexican Axolotl have produced no hereditary effect. The fact, however, that Axolotls require special treatment to induce metamorphosis seems to show that they have distinctly less congenital tendency to metamorphosis than larvae of the same species, Amblystoma tigrinum, in other parts of North America, and this difference must be attributed to the inherited effect of the conditions. The most important of these conditions seems to be abundance of oxygen in solution in the water, and the next in importance abundance of food in the water. Recently it has been shown that the metamorphosis may be induced by feeding Axolotls on thyroid gland. But there is no reason to suppose that a congenital defect of thyroid arising as a mutation was the original cause of the neoteny, i.e. the peisistence of the larval or aquatic, branchiate condition. Such a supposition would imply that the association between Axolotls and the peculiar Mexican lakes, supplied with oxygenated water by springs at the bottom, was purely accidental. Moreover, there is no evidence that there is any deficiency of thyroid in the Axolotl. The secretion of the thyroid gland is necessary for the normal growth and development of all Vertebrates, and we are only beginning to understand the effects of defect or excess of this secretion. There is nothing very surprising in the fact that excess in the case of the Axolotl causes the occurrence of the metamorphosis which had already in numerous experiments been produced by forcing the animals to breathe air.

Metamorphosis, as in the development of gill arches and gill slits in the embryos of Birds, Reptiles, and Mammals, exhibits a recapitulation of the stages of evolution of certain organs. But in the case of other organs the absence of recapitulation is remarkable by contrast. If, as I believe, the development of lungs and disappearance of gills was directly due to the necessity of breathing air, it is difficult to avoid the conclusion that the terrestrial legs were originally evolved from some type of fishes' fins by the use of the fins for terrestrial locomotion. Yet neither the amphibian larva nor the embryo of higher Vertebrates develops anything closely similar to a fin. There is no gradual change of a fin-like limb into a leg, but the leg develops directly from a simple bud of tissue. The larva of the Urodela is probably more primitive than the tadpole of the Frogs and Toads, and in the former the legs develop while the external gills are still large, long before the animal leaves the water.

It is possible that the limbs were transformed to the terrestrial type before the animal itself became terrestrial, the habit of swimming having been partly abandoned for that of crawling or walking at the bottom of the water, and the tail being used merely for swimming to the surface to obtain air. But the condition of the Dipnoi, which possess lungs but do not walk on land, does not support this supposition, for they possess fins which are either filamentous or fin-like, having a central axis with rays on each side. There can be little doubt that the digits of the terrestrial limb are homologous with endoskeletal fin-rays, but the evolution of the axis of the limb is not to be ascertained either from development or palaeontology. The absence of metamorphosis here may perhaps be due to the fact that the lateral fins ceased to function in the earlier aquatic stages, only the caudal fin being used for swimming. If this were the case the absence of metamorphosis in the legs is itself an adaptation, the disuse of the paired limbs in the larva having caused the earlier fin-like stages of these limbs to disappear, while the terrestrial leg was developed later by heredity, just as the legs have disappeared in the larvae of many insects, though fully developed in the adult.

Metamorphosis of structure in Amphibia and in Flat-fishes corresponds to the change of conditions of life in the free-living animal. In the case of the eyes of the Cave-fishes the conditions in respect of absence of light are constant throughout life, and we find only an embryonic development of the eye taking place by heredity. The question arises whether, when there is no embryonic recapitulation, it must be concluded that apparent adaptations are due to mutation and not to function or external conditions. One case of this kind is that of the limbs of Snakes, where, if we except the vestiges of hind limbs in the Pythons, there is no trace of limbs either in the embryo or after hatching. There are several similar cases among Reptiles and Amphibia. The Slow-worm (Anguis fragilis) is limbless, and so are the members of the sub-class Apoda among the Amphibia. In these also rudiments of limbs are entirely absent in the embryos or larval stages. Considering the recent evolution of Snakes as compared with the origin of lungs and loss of gills and gill slits in terrestrial Vertebrates in general, we have here a remarkable contrast which shows in the first place the difference resulting when the change in habits and conditions in the one case takes place from one stage of life to another, and in the other case the new habits are constant throughout life from the moment of hatching. It seems to me that in the present state of our knowledge we cannot form a decisive opinion on the question whether the absence of limbs in such cases is the result of mutation or of disuse—that is, absence of functional stimulation.

The power of flight is an excellent example of adaptation. It has been evolved independently in Pterodactyls, Bats, and Birds. In the two first groups, and to a slight degree in the third, the expanse of the wing is formed by an extension of the skin into a thin membrane, supported by the fore-limbs. It is not necessary to argue in detail that the evolution of this membrane and of the modifications of bones and muscles by which it is supported and moved, can be satisfactorily explained on the theory that modifications due to mechanical and functional stimulation are ultimately inherited. In birds, however, the surface of the wing is supplied chiefly by feathers, and consideration of the matter affords no reason for supposing that the evolution of feathers was due to any external or functional stimulation. It is often stated that the feathers of birds are a modification of the epidermic scales of reptiles, but investigation does not fully confirm this statement. The reptilian scales are retained on the tarso-metatarsal region of the leg in the majority of birds, and it would be expected, if the view just quoted were correct, that a transition from scales to feathers would be visible at the ankle-joint. This, however, is not the case. In fowls some breeds have scaly shanks and others feathered. In those with scaly legs I have found cases in winch, in the chicks, there were two or three very minute feathers, and I have examined these microscopically by means of sections of the skin. The result was to show that the minute feathers were not a prolongation of the tips or edges of the scales, but arose from follicles between the scales. The scale is flat and is a fold of the epidermis not arising from an invaginated follicle. The feather, on the other hand, is a tubular structure arising from a papilla at the base of a deep follicle extending inwards from the surface of the skin. As the feather grows the papilla grows with it. This papilla consists of vascular dermal, i.e. mesodermic tissue, and if the feather is pulled out during growth bleeding occurs. The epidermic horny tube splits posteriorly towards the apex of the feather, and is divided into rachis and barbs, and thus the dermal tissue within, by this time dead and dry, is exposed and is shed. Every feather is in fact an open wound, and is perhaps the only other case, in addition to that of the antlers of stags, in which vascular mesodermic tissue is normally shed in such considerable quantities. When the development of the feather is complete, growth gradually ceases, the proximal part of the feather remains tubular and does not split, and the vascular tissue within dies, shrivels, and dries up, forming the pith of the quill When the papilla recommences to grow the old feather is pushed out, and this process causes the moult. It would appear, therefore, that the feather must have been evolved, not by a continuous modification from the scale but by a development of a new kind between the scales. I have been unable to discover hitherto any evidence suggesting an external stimulus which could cause this remarkable process of development in feathers, or indicating that the function of flight would involve such a stimulus. For the present, therefore, we must conclude that feathers are not an adaptation, and not due to somatogenic modification, but must be result of a gametogenic mutation.

Feathers, having been evolved, served in the wings and tail as important organs of flight. There is reason to believe that, once present, the growth of feathers was modified greatly by the degree of stimulation applied to the papillae at roots by the movement and bending strain of the feathers. The modification of the hones and of the wing, shoulders, and sternum by the functional stimuli involved in flying are obviously adaptations, and in my opinion are only to be explained as the hereditary effects of functional stimulation, like all skeleto-muscular adaptations. The strains produced in bones by muscular contraction produce hypertrophy of the part of the bone to which the muscles are attached and thus we can understand the origin of the carina of the sternum in flying birds, and its absence in flightless forms. In bats and in pterodactyls also the sternum is produced into a carina along the median line. The reduction of the digits of the wing in birds to three, with the bones firmly united together, would follow from their use in flight and their disuse as digits, and it would seem, from the fact that the flight-feathers must have been always on the posterior edge of the wing, and that the ulna is larger than the radius, that the three digits which have persisted are the 3rd, 4th, and 5th, and not the 1st, 2nd, and 3rd as usually taught. A comparison of the hind-limbs of birds with those of bats and pterodactyls suggests strongly that the patagium flyers have arisen from arboreal or climbing animals, while the birds arose from terrestrial forms which acquired the bipedal habit, as certain reptiles have. An arboreal animal would necessarily use all four limbs, as climbing animals actually do. The wings of birds, on the other hand, would have arisen, from the endeavour to increase speed by movements of the fore-limbs. The perching birds would therefore have arisen by later adaptations after the power of flight had been evolved.

Complete recapitulation does not occur in the development of the digits of the wing. Only a rudiment of a fourth digit has been found in the embryonic wing, not, as might be expected, rudiments of five digits of which two disappear. The metacarpals are free, not united as in the adult, and there are separate distal carpals, which in the adult are united with the metacarpals. In other respects the modifications of wings and sternum are so obviously adaptive that it is difficult to believe that the reduction of digits was not due to disuse. This is another of those cases in which the function to which structure is adapted is constant from the beginning of independent life to the end, and there is some ground for believing that in course of time in such cases embryonic recapitulation may be much diminished or disappear. The period of time since birds were first evolved is in all probability immensely greater than that which has elapsed since the blind fish, Amblyoysis, was modified by cave-life, so that we can understand why the eye is developed to a certain stage in the embryo of the blind fish, although it lives in darkness all its life, while embryonic recapitulation in the wing of the bird is very incomplete.

In another class of adaptations the embryonic or larval stage is adapted to new conditions, while the adult condition is either less changed or not changed at all. One of the most obvious examples of this is the allantois in the Amniota. The embryos of Reptiles, Birds, and Mammals all develop two embryonic or foetal membranes, the amnion and the allantois. Of the function or origin of the amnion little is known: to state that it is protective affords little explanation. It seems possible that it is merely the mechanical result of the weight of the embryo and the development of the allantois. The latter is a precocious hypertrophy of the cloacal bladder found in Amphibia, with the function of embryonic respiration. In the water the amphibian larva respires by means of gills and gill slits. In adaptation to terrestrial life it is necessary, if the free aquatic larval stage is to be eliminated, that the embryo should be able to breathe air before hatching. Various Amphibia show how this requirement was met in various ways. In the South American tree-frogs of the genus Nototrema the eggs are developed in a dorsal pouch of the skin of the female, and within this pouch the respiration of the embryo is carried on by a membranous expansion of the second and third external gills on each side. In the Reptilia the bladder is expanded for the same function, and absorbs oxygen and gives off carbon dioxide through the pores of the shell. It is impossible to reconcile the conception of mutation with the adaptive relation between this allantois and the expulsion of the egg enclosed in a shell on land. The transition probably came about gradually from the deposition of the eggs in moist places but not in water. In the midwife toad (Alytes obstetricans) the male carries the eggs about attached to his legs, respiration is effected by enlarged external gills, and the larvae are hatched in water. In the ancestral reptiles external gills may have helped at first, until by the enlargement of the bladder they were rendered unnecessary. In all such cases the absorption of oxygen must be regarded as the stimulus which caused the enlargement of the respiratory membrane. As the allantois could not be absorbed or retracted again into the abdomen, the umbilicus was evolved—that is to say, the scar formed by the union of the folded edge between the body wall and amnion surrounding the stalk of the allantois. It would he difficult for a mutationist to explain how a mutation should affect the development of the cloacal bladder to such an enormous degree, just when it was required for embryonic respiration, and cause the sides of the body to unite ventrally at the time of hatching, cutting off the allantois and the amnion.

T. H. Morgan [Footnote: A Critique of the Theory of Evolution, p.18.] states that a mutation of gametic origin may affect any stage in the development of the individual. This may be true when there are already distinct stages in the life history. The more important question is whether distinct stages can be caused by mutation. It is true that in heterozygous individuals characters may develop more fully in the adult stage than in the young. But when we find different stages evidently adapted to different modes of life, it is impossible to explain them by mutations affecting different stages of life. In such cases as the larval stages of Insects we find the larvae have become adapted to new habits while the adults have remained unchanged, or have evolved quite independent adaptations. For example, the adults in the chief orders of Insects have the typical three pairs of legs, while the maggots or grubs of the Diptera or Hymenoptera have no legs at all, the caterpillars of Lepidoptera have evolved pseudo-legs on the abdomen, and the larvae of Coleoptera have the ordinary legs and no more. This is the reverse of recapitulation: in the case of legless maggots, and caterpillars with pro-legs, the adult is more similar to the ancestor than the larva. But the same principle holds, that where functions and habits are different, there organs are different. No mutationist has yet produced by breeding experiments a caterpillar without the three pairs of thoracic legs and yet developing into a moth that had normal three pairs. Morgan, with all his mutations of the adult Drosophila, says nothing of mutants possessing legs. The only rational conclusion is that legless larvae have lost the disuse, since those larvae which are destitute of legs do not go in search of food but either live in the midst of it or are fed by others, and that the pro-legs of the caterpillar have been developed by the muscular action of the insect in clinging to leaves. Here again the hormone theory, although we cannot pretend to understand the matter completely, helps us to form a conception of the process of heredity and evolution. The disuse of legs in the larva affects the determinants, so that they remain inactive in the presence of the hormones produced in the body generally in this stage. In the adult stage activity of the legs produces hormones which influence the same determinants in the gametes to develop legs, but again in the presence of the different hormones which are present in the body generally in the adult stage. As the habits of larva and adult became more specialised and contrasted, the change became less and less gradual, and the intermediate stage, not being adapted to any transitional mode of life, became an inactive pupa in which the adult organs develop.

In conclusion I will briefly consider the attempts which have been made to prove the influence of somatic modifications or characters on the gametes by direct experiment. The method of Kammerer of inducing changes of habit or structure by conditions, and then showing that the change is in some degree inherited, has already been mentioned. One obvious criticism of this evidence is that it seems to prove too much, for it is difficult to believe that a change produced in individuals would show so much hereditary effect in their immediate offspring. Two other methods are conceivable by which the influence of somatic hormones might be evident. One of these is to graft ovaries or testes from one animal into another which possesses a certain somatic character, and then to see if the offspring produced from these gonads shows any trace of the character of the foreign soma in which it was nourished. C. C. Guthrie [Footnote: Journ. Exper. Zool. (1908), v.] claimed to have done this in his experiments on hens. He grafted the ovaries of two Black Leghorn pullets into two White pullets of the same breed, and vice versa. The black and the white birds bred true when mated to cocks of their own colour. The black hen with white ovary mated with black cock produced four black chicks and two black chicks with white legs, the white hen with black ovary mated with white cock produced some white chicks, some black and some white with black spots. This is held to prove that the transplanted ovaries were functional, because they produced evidence of the character originally belonging to them. On the other hand, the black hen with white ovary mated with white cock produced nine white chicks, and eleven chicks which were white spotted with black, and the white hen with black ovary mated with black cock produced not black chicks but white chicks spotted with black. This was held to prove that the somatic characters of the "foster mothers" were transmitted.

Davenport repeated Guthrie's experiments on different fowls, grafting the ovary from a cinnamon-coloured hen into a white hen, and mating her with a cinnamon-coloured cock. The chicks were exactly similar to those obtained from crossing such a cock with a normal white hen, and Davenport concludes that the engrafted ovary was not functional but had degenerated. It is known to be almost if not quite impossible to remove the ovary completely from a hen, owing to its close attachment over the great post-caval vein. At the same time it is difficult to see how Guthrie could have obtained black and spotted chicks from a white hen mated with, a white cock if the grafted ovary from a black hen had not been functional. One point which Guthrie does not mention, and of which apparently he was not aware, is that the white of the White Leghorn is dominant to colour, the heterozygotes not being pure white but white with spots. Thus when he mated a black cock with a white hen with grafted ovary and obtained spotted chicks, this would have been the result if the original white ovary was functional. None of his results prove conclusively the influence of the soma of the hen into which ovaries were grafted, but would all be explained if some eggs were derived from the part of the original ovary not removed in the operation, and others from the grafted ovary.

The grafting of ovaries in Mammals has often been tried, but very rarely with success. The introduced ovary usually dies and is absorbed. C. Foa [Footnote: Arch. Ital. de Bid. (1901), Tome xxxv.] states that he made bilateral grafts of ovaries from newborn rabbits into adult rabbits, and two months after the operation one of the operated females was fecundated and produced five normal young. In other cases he placed ovaries from new-born young in positions far from the normal position, such as the space between the uterus and bladder, and in one case the female so treated became pregnant, and when killed had a single embryo in one uterus and no trace of the original ovaries in the normal position. But Foa was not investigating the influence of somatic characters on ova in the grafted ovaries, and does not even mention the characters or breed of the rabbits he used or of the young which were produced from the grafted ovaries. Castle [Footnote: W. E, Castle and J. C. Phillips, On Germinal Transplantation in Vertebrates, Pub. Carnegie Institution in Washington (1911), No. 144.] carried out seventy-four transplantations of ovaries principally in guinea-pigs. Out of all these only one grafted female produced young. In this case the ovaries of two different black guinea-pigs about one month old were grafted into an albino female about five months old. After recovery the grafted female was kept with an albino male. She produced six young in three pregnancies, first two, then one, and lastly died with three foetus in the uteri. All these were black, with some red hairs among the black. One of the first two young had a white forefoot. In this case black is dominant, and therefore there is nothing extraordinary in the offspring from a black grafted ovary being black. The presence of red hairs and a white foot is no evidence of the influence of the foster soma, but is due to imperfect dominance. When the same male was mated with a normal black female the offspring were black with red hairs interspersed.

All these experiments are open to the following criticism. It has been the main argument of this volume that there are two distinct kinds of characters in all organisms—namely, those of somatogenic origin and those of gametogenic origin. Theory supposes that somatic modifications by means of hormones affect the determinants in the gametes. But it is obvious that the black and white of Leghorn fowls and of guinea-pigs are gametogenic characters, and are strongly established in the gametes of their respective varieties. It is not even certain that the black or white hair or feathers are giving off special hormones which would or could influence the gametes. The hormone theory only postulates such influence from hormones issuing from tissues modified by external stimuli. It is quite certain that the black colour in Leghorns or guinea-pigs is not due to any external stimulus or influence. The experiments therefore are entirely irrelevant to what has been called the inheritance of acquired characters. All that they can be said to prove is that an albino soma does not convert ingrafted ova of black race into ova carrying the albino character.

It is probably impossible to prove experimentally the influence of a modified soma in one generation. I have endeavoured to find a case which would not be open to the above criticism—that is, to find a character which could be considered somatogenic and which was absent in a closely allied variety. Most of the characters in domesticated varieties are obviously gametogenic mutations, but the lop-ear in rabbits may be, partly at least, somatogenic. Since many breeds have upright ears, we cannot say that disuse of the external ear has produced lop-ears in domesticated rabbits generally, but in lop-eared breeds the ears are much enlarged; and though this may be gametogenic, the increased weight may have been the cause of the loss of the power to erect the ears. I therefore tried grafting ovaries from straight-eared females into lop-eared individuals. The operation was perfectly successful in seven specimens—that is to say, they recovered completely and lived for many months, up to a year or more afterwards, but none of them became pregnant. When killed no trace of ovary was in any of them; in every case it had been completely absorbed, and the uteri and vagina were diminished in size and anaemic. For grafting I used ovaries from young rabbits of various ages from seven days to six weeks or more, but all were equally unsuccessful. Satisfactory evidence by direct experiment of the inheritance of somatogenic modifications due to external stimuli cannot be said to have been yet produced, and, as I have shown, such evidence from the nature of the case must be very difficult to obtain. The indirect evidence, however, which has been considered in this volume is too strong to be ignored—namely, the case of Japanese long-tailed fowls, that of colour on the lower sides of Flat-fishes, and the similarity of the congenital development of the antlers in stags, to the generally admitted effects of mechanical stimulation and injury on the skin and superficial bones of Mammals.

The general conclusions which are logically to be drawn from our present knowledge with regard to the problems of heredity and evolution in animals are in my opinion as follows:—

1. All attempts to explain adaptation by gametogenic mutations, or changes in gametic factors or 'genes,' have completely failed, as Bateson himself has admitted.

2. The facts discovered concerning mutations and Mendelian heredity harmonize with the nature of the majority of specific and varietal characters, and with the diagnostic characters of many larger divisions in classification.

3. Some of the most striking cases of adaptation, such as the organs of respiration and circulation in terrestrial Vertebrates, and the asymmetry of Flat-fishes, are developed in the individual by a metamorphosis which is generally regarded as a recapitulation of the ancestral evolution. No cases of mutation or gametogenic variation hitherto described exhibit a similar metamorphosis or recapitulation.

4. Secondary sexual characters, usually in the male sex, correspond in their development with the development of maturity and functional activity in the gonads, and it has been proved that the latter influence the former by means of 'hormones' or internal secretions. The evidence concerning sex and sex-linked characters and the localisation of their factors in the chromosomes of the gametes has no bearing on the action of hormones.

5. The facts concerning the action of hormones are beyond the scope of current conceptions of the action of factors or genes localised in the gametes and particularly in the chromosomes. According to these conceptions, characters are determined entirely by the genes in the chromosomes, whereas in certain cases the development of organs or characters depends on a chemical substance secreted in some distant part of the body.

6. It was formerly stated that no process was known or could be conceived by which modifications produced in the soma by external stimuli could affect the determinants in the gametes in such a way that the modifications would be inherited. The knowledge now obtained concerning the nature and action of hormones shows that such a process actually exists, and in modern theory real substances of the nature of special chemical compounds take the place of the imaginary gemmules of Darwin's theory of pangenesis or the 'constitutional units' of Spencer.

7. The theory of the heredity of somatogenic modifications by means of hormones harmonises with and goes far to explain the facts of metamorphosis and recapitulation in adaptive characters, and also the origin of secondary sexual characters, their correlation with the periodical changes in the gonads and the effects of castration. At the same time there are some somatic sex-characters, e.g. in insects and birds, which do not appear to be correlated with changes in the gonads, and which are probably gametogenic, not somatogenic in origin.

8. The theory of the heredity of somatogenic modifications is not in opposition to the mutation theory. The author's view is that are two kinds of variation in evolution, one somatogenic and due to external stimuli, acting either directly on passive tissues or indirectly through function, and the other gametogenic and due to changes in the chromosomes of the gametes which are spontaneous and not in any way due to modifications of the soma. Adaptations are due to somatogenic modifications, non-adaptive diagnostic characters to gametogenic mutations. It is a mistake to attempt to explain all the results of evolution by a principle. There are two kinds of congenital, constitutional or hereditary characters in all organisms, namely, the adaptive and the non-adaptive, and every distinct type in classification exhibits a combination of the two. To assert that all characters are adaptive is as erroneous as to state that all characters are blastogenic mutations, and therefore in their origin non-adaptive.

9. Finally it may be urged, although the question has not been directly discussed in this volume, that no biologist is justified in the present state of knowledge in dogmatically teaching the lay public that gametogenic characters are alone worthy of attention in questions of eugenics and sociology. Hereditary or constitutional factors are of course of the highest importance, but there exists very good evidence that modifications due to external stimulus do not perish with the individual, but are in some degree handed on to succeeding generations, and that good qualities and improvement of the race are not exclusively due to mutations which are entirely independent of external stimuli and functional activity. It is important to produce good stock, but it is also necessary to exercise and develop the moral, mental, and physical qualities of that stock, not merely for the benefit of the individual, but for the benefit of succeeding generations and to prevent degeneration.



INDEX

Abraxas groussularioun and lacticolor Adaptations, origin of; evolution of Agonus entaphractus Albinism Allantois Allurements Alytes obstetricans Amblyopsis, eyes of Amblystoma tigrinum Amnion Anableps tetrophthalmus Anas boscas, crosses of Anas tristis, crosses Ancel and Bouin Anguis fragilis Antilocapra Antirrhinum, crossing of Antlers of stags Ants, heredity of sex in Aphidae, heredity of sex in Apoda Axolotl, albino; metamorphosis; influence of thyroid feeding

Barred plumage in fowls Basoh Bateson Bees, heredity of sex in Bernard, Claude Berthold, A. A. Biedl and Konigstein Bionomies Blindness in cave animals Bombyx mori Boring, Miss Born and Fraenkel Brachydactyly Bresslau Brown-Sequard Buehler

Cambarus, males of Capons Castle, experiments in grafting; on sex Castration; in ducks; of frog; of Lepidoptera Cats, heredity of colour in Cave animals, absence of pigment Cephalopoda Cetacea, absence of scrotum Chelonia Chologaster agassixii Chromosomes; in mutations Clevelandia Colaptes Colour-blindness; heredity of Colours, origin of, in domesticated breeds Comb of fowls, uselessness of Corpora lutea, evolution of; in viviparous lower vertebrates; origin of Corystes cassivelaunus Courtship, organs of Criss-cross inheritance Crossing over Cryptorchidism Cuttle-fishes Cyclostomes, absence of corpora lutea in Cytology Cytoplasm, in heredity

Dafila acuta crosses Daphnia, heredity of sex in Darwin Dasyurus; corpora lutea; lactation Davenport Determinants Determination of sex Dipnoi, fins Dog-fishes, oviparous and viviparous Dominant characters, origin of Doncaster; on heredity in cats Drosophila, blind mutation, heredity of sex, mutations Ducks, crosses of Dutch rabbit

Earthworms, sex in Eclipse plumage Eigenmann Eimer Elasmobranchs; corpus luteum in Elephants, testes Eugenics Eunuch Evolution, evidence of

Factors, origin of Feathers, evolution of Flat-fishes, mutations of Flight, evolution of Flounder Foa, on lactation; on grafting ovaries Foges Fowls, castration of; origin of breeds Fractionation of Mendelian factors Fraenkel Frog, thumb-pad

_Gallus bankiva_ Gates, Dr. R. Ruggles Geddes and Thomson Gemmules Genital ducts _Gigas, Oenothera_ _Gillichthys Gipsy moth Goltz and Ewald Gonads, hormones of Goodale, H. D. Grafting, of ovaries or testes Graves' disease Gudernatsch Guthrie, C. C. Gynandromorphism

Haemophilia Hanau Hegner Herdwick sheep, castration in Heredity; and sex Hermaphroditism Hill, J. P. Horns Houssaye

Inachus scorpio Insects, heredity of sex in Interstitial cells Intromittent organs

Japanese long-tailed fowls; artificial treatment of

Kammerer Kellog Kopec

Lactation, dependence on stimulation, in males; regulation of Laevifolia, Oenothera Lamarck Lamarckian theory Lane-Claypon, Miss; and Starling, on ovaries of rabbit Larvae of insects Lata, Cenothera Leghorn, White Lemon-dab Leopold and Ravana Lepidoptera, castration in Leptinotarsa Limantria dispar Limon Linnaeus Lode Loeb, on "blind fish; on blindness in cave animals; on tadpoles and thyroid Lop-eared rabbits, grafting experiments Lotsy, Professor; on crossing Lutein, of corpora lutes

Male characters in female Mallard crosses Mammary glands; origin of rudimentary in male Marshall; and Jolly Marsupials, relation of foetus to pouch; scrotum of Masked crab Meisenheimer; thumb-pad of frog Mendel's Principles of Heredity Mendelism; and castration Menstruation Metamorphosis; in Flat-fishes; causes of; and hormones; and diagnostic characters Michaux, Midwife toad, Milk glands, Mole, eyes of, Monotremata, origin of milk glands, Morgan, T. H., on blindness in cave animals, on mutations, on sex:, on sex-linked heredity, on sexual dimorphism in Drosophila, on variation, Mutations, in antlers,

Natural selection, Nuptial plumage, Nussbaum, Nyssia zonaria

O'Donoghue, development of milk glands, _OEnothera_, mutations, _grandiflora_, lata_, _Lamarckiana_, Onagra, species of, _Origin of Species_, Darwin's, _Ornithorhyncus_, corpus luteum Orthogenesis, Otariidae, scrotum, Ovaries, position of, Ovary, in birds, Ovulation,

Pangenesis, Parthenogenesis, Parturition, Pearson, Karl, Pheasant, male, gynandramorphism in Phillips, John C., Philosophie Zoologique Phoeidae, testes, Physiology of Reproduction, Picotee Sweet Pea, Pigeons, Pigment, absence in cave animals, Pile fowls, Pintail duck, crosses, Plaice, Pleuronectes flesus, glacialis, platesca, Plymouth Rock fowl, Pole-dab, Poll, Preformation, Problems of Genetics, Prong buck, Pro-oestrus, Proteus, eyes of, Prototheria, milk glands in,

Rabbits, lactation in, Recapitulation, absence of, and mutations, Reptiles, corpora lutea in, Reversal, in Flat-fishes, Rhinoderma darwinii, Ribbert, Rieger, Rodents, testes, Romanes, GJ Roentgen rays, effect on testes, Rose comb, in fowls, Rotifers, heredity of sex in, Rubricalyx, Oenothera, Rubrinervis, Oenothera,

Sacculina, Salamanders, transplantation of eye, Sandes, Schuster, Edgar, Scrotum, origin, of, Sea-horse, Secondary sexual characters, Selheim, Semilata, Oenothera, Sertoli's cells, Sex, chromosomes; Mendelian theory of, Sex-Linked heredity, Sexual Dimorphism, Sexual dimorphism, in Rajidae, in Plaice, Shattock and Seligmann, Silkworm, Silky fowl, plumage of, Sirenia, absence of scrotum, Slow-worm, Smith, Geoffrey, Snakes, absence of limbs, Sociology, Somatic sexual characters, Species, conception of, origin of, characters of, sterility and hybridism, Spermatogenesis, in man, Starling and Lane-Claypon, on lactation, Steinach, heredity of milk glands, Sternum, carina of, Swallows, Sweet Pea, Swifts,

Tadpoles, effect of thyroid in Tandler and Gross Taxonomies Teleosteans; corpora lutea in; ovarian follicles Testes, descent of Tetraploidy Thayer Thumb-pad of frog Thyroid-gland feeding Tortoise-shell colour in cats Tosa fowls, Japanese Transplantation of gonads Typhiogobius

Uhlenhuth Urodela, larva

Variations Vespa vulgaris; germanica Vries, De

Wallart Wasps; heredity of sex in Weapons, organs used as Weismann Whale, paddle of White Leghorn, crosses Wilson, E. B. Wing, development of Winiwarter, von Witch Wood, T. B., on crossing of sheep Woodland, W. Woodpecker

X chromosome

Zeugopterus Zoaea

THE END