Morgan evidently does not realise that the Birds and Reptiles must have been derived from Amphibia, and that the embryo Reptile or Bird with gill slits and gill arches is merely a tadpole enclosed in an egg shell. The Frog in its adult state differs much from a Fish, while the larva in its gill arches and gill slits resembles a Fish. Morgan contends that the new characters do not add themselves to the end of the line of already existing characters. But in the case of the Frog this is exactly what they have done. The existing characters were in this case the gill arches and slits. Those who believe in recapitulation do not suppose that the animal had to live a second life added on to the life of its ancestors and that the new characters appeared in the second life. They believe that in the ancestor a certain character or general structure of body when developed persisted without change throughout life like the gill arches and slits in a Fish. At some stage of life before maturity this character underwent a change, and in the descendants the development of the original character and the change were repeated by heredity. There is no 'mysterious pushing back of adult characters into the embryo,' although it is possible or even probable that in some cases the change gradually became earlier in the life history: it is the new character which is pushed back, not the adult character of the ancestor.
It is perfectly true, as Morgan says, that new characters which arise as discontinuous variations—in other words, those kinds of variation which are called mutations—do not add themselves to the line of already existing characters, but 'change the adult characters without as it were passing through and beyond them.' The mutations which Morgan describes in his own experiments on Drosophila illustrate this in every case. In no case is the original organ or character, e.g. wings, of the normal Fly first developed and then changed by a gradual continuous process into the new character. It might perhaps be said that this took place in the pupa, but that seems impossible, for the complete wing is not fully developed in the pupa. The same truth is equally apparent in the mutations described in OEnothera. It follows, therefore, that none of the evolutionary changes which have produced what are called recapitulations can have been due to changes of that kind which is known as mutation.
The abnormalities in Pleuronectidae to which I have referred are of the kind usually regarded as due to arrested development. But closer consideration gives rise to doubt concerning the validity of this explanation. It might be supposed that the attached base of the dorsal fin is unable to extend forward because the eye on the edge of the head is in the way, but if the metamorphosis is arrested, why should the fin grow forward in a free projection? I have described a very abnormal specimen of Turbot in a paper communicated to the Zoological Society of London, [Footnote: Proc. Zool Soc., 1907.] and in that paper have discussed other possible explanations of these mutations. In the specimen to which I refer the pigmentation instead of being present on both sides was reversed: the lower side was pigmented from the posterior end to the edge of the operculum (Plate II, fig. 2), while the upper side was unpigmented excepting a scattering of minute black specks and a little pigment on the head (Plate II., fig. 1).
[Illustration: PLATE II, Fig. 1 and Fig. 2,
Abnormal Specimen Of Turbot]
I have suggested that the explanation here is that in the zygote the primordia of a normal body and a reversed head have been united together. We may suppose that different parts of the body are represented in the gametes by different determinants or factors, and therefore it is possible that these factors may be separated. In the specimen we are considering the body is normal or nearly so, with the pigmentation on the left side, which is normal for the Turbot, while the head has both eyes with some pigment on the right side and the left side unpigmented. Reversed specimens occasionally occur in many species of Pleuronectidae, and if the determinants for a reversed head and a normal body were united in one zygote, the curious abnormality observed might be the result. It is just a possibility that if this fish which was only 4.4 cm. long had lived to adult size, the upper side would have become pigmented under the influence of light, while the strong hereditary influence would have prevented the disappearance of the pigment from the lower side. In that case the adult condition would have been similar to that of ordinary ambicolorate specimens, but reversed, with eyes on the right side instead of the left. Other explanations of the more frequent ambicolorate mutation are possible: the body may consist of two left sides instead of a left and right, joined on to a normal head. But the first suggestion seems the more probable, as two rights or two lefts would not be symmetrical. Supposing the head and body not properly to belong to each other, one being reversed and one normal, we can in a way understand why the dorsal fin does not form the usual connexion with the edge of the head, because the determinants would not be in the normal intimate relation to each other. In thus writing of reversed and normal it must be understood that the former word does not mean merely turned over, for in that case right side of the body would be joined to the left side of the head, and the dorsal fin would be next to the ventral side of the head, which is not the case. What is meant is that a left side of the body which is normally pigmented is joined to a left side of the head which instead of having both eyes has neither, the two eyes being on the right side of the head which is joined to the right side of the body, and this is normal and unpigmented. The dorsal fin belonging to the normal sinistral body would therefore have a congenital tendency in the metamorphosis to unite with the head on the outer side of the original lower or right eye after it has moved to the left side. Actually, however, in this abnormal specimen it finds itself on the outer side of the left eye which has passed to the right side, and it has no tendency to unite with this part of the head. At the same time it has no tendency to bend over at an angle to reach the outer side of the right eye, and therefore it grows directly forward without attachment to the head at all.
It will be seen, therefore, that what is changed in relative position in these mutations is not the actual parts of the body, but merely the characters of those parts. In a sinistral Flat-fish, whether it is normally sinistral like the Turbot or abnormally like a 'reversed' Flounder, the viscera are in the same position as in a dextral specimen: the liver is on the left side, the coils of the intestine on the right. Thus in a reversed or sinistral Flounder, which is normally dextral, the left side which is uppermost is still the left side, but it has colour and two eyes, whereas in the normal specimen the right side has these characters and not the left. Thus we are forced to conceive of the determinants in the chromosomes of the fertilised ovum which correspond to the two sides of the body, as entirely distinct from the determinants which cause the condition or 'characters' of the two sides, unless indeed we suppose that determinants of right side with eyes and colour occur in some gametes and of right side without eyes and colour in others, and vice versa, and that homozygous and heterozygous combinations occur in fertilisation. On this last hypothesis the mutation here considered might be a heterozygous specimen, with the dextral condition dominant in the head and the sinistral in the body. Or it might be somehow due to what Morgan and his colleagues have called crossing over in the segregation of heterozygous chromosomes, so that a part corresponding to a sinistral body is united with a part corresponding to a dextral head.
My conclusion from the evidence is that any process of congenital development may in particular zygotes exhibit a mutation, a departure from the normal. We need not use the term heredity at all, or if we do, must remember that in the present argument it does not refer to any transmission from the parent. The factors in the gametes of the normal Flat-fish egg cause the normal metamorphosis to take place after the larval symmetry has lasted a certain time. In occasional individuals the factors whatever they are, portions of the chromosomes or arrangement of the chromosomes or anything else, are different from those of the normal egg, and in consequence the abnormalities above described are developed. But the chief fact which I cannot too strongly emphasise is that the development of the abnormality from the symmetrical larva is direct, whether it is merely an arrest of development or an abnormal combination of reversed and normal parts. The abnormal development is not due to a change occurring after the normal asymmetry has been developed. These abnormalities are true mutations.
The evolution of the normal Flat-fish, on the other hand, was obviously due to a change of a different kind. Here we are dealing with the change from a symmetrical fish to the asymmetrical. Judging from what takes place in other mutations, it was quite possible for asymmetry to have developed directly from the egg, in consequence of some difference in the chromosomes of the nucleus. It has been shown that placing a fish egg for a short time in MgCl[2] [Footnote: Stockard, Arch. Eut. Mech., xxiii. (1907).] causes a cyclopean monstrosity to be developed in which the two eyes are united into one: but the two eyes do not develop separately first and then gradually approach each other and unite, the development of the optic cups is different from the first. In the normal Flat-fish the evolution that has occurred is the original development of the symmetrical fish, and the subsequent continuous gradual change in eyes, fin, and colour to the adult Flat-fish as we see it. All the evidence accumulated by the experiments and observations of mutationists and Mendelians goes to prove that this change is of an entirely different kind from those variations which are described as mutations, or as loss or addition of genetic factors.
This being the case, we have to inquire what is the explanation of the evolution of the normal metamorphosis.
The important fact is that the original symmetrical structure of the larva and the asymmetrical structure of the adult Flat-fish correspond to the different positions of the body of the fish in relation to the vertical, the horizontal ground at the bottom of the water, and incidence of light. The larva swims with its plane of symmetry vertical like most other fishes; its locomotion requires symmetrical development of muscles and fins; the two sides being equally exposed to light, it requires an eye on each side, and the pigment on each side is also related to the equal exposure to light. The adult lying with one side on the ground has its original plane of symmetry horizontal and parallel to the ground, and only the other side exposed to light, and on this side only eyes and colour, i.e. pigment. The change of structure corresponds with the change of habit. It consists in the change of position of the lower eye, the extension of the dorsal fin forwards, and the disappearance of pigment from the lower side. In the actual metamorphosis these changes take place as the skeleton develops, before the hard bones are fully formed, while the fish is still small, but the young Turbot reaches a much larger size before metamorphosis is complete, namely, about one inch in length, than the young Plaice or Flounder. It is of little importance to consider whether at the beginning of the evolution the change of position occurred late or early in life. It may have become earlier in the course of the evolution. The important matter is to consider the evidence in support of the conclusion that the relation to external conditions has been the cause of the evolutionary change. We have already seen that the nature of the change and the relation of the change of structure to the change of conditions necessarily tend to the inference that the latter is the cause of the former. But we have to consider the particular changes in detail.