Hormones and Heredity Part 2

The Dab, which occurs in the same waters as the Plaice, has the spines more spinulated than any of the forms of plaice above mentioned, therefore the absence or slight development of spinules in the typical Plaice is not explained by physical conditions alone. Freshness of water again will not explain the difference of the structure and distribution of scales in Flounder and Plaice, considering the variety of squamation in fishes confined to fresh water. Still less can we attribute any of the peculiarities of scales to utility. We can discover no possible benefit of the condition in one species which would be absent in the case of other species. We can go much further than this, and maintain that there is no reason to believe that scales in general in Teleosteans, or any of their various modifications, are of special utility: they are not adaptive structures at all, although of great importance as diagnostic characters.

It may be urged that in some cases, such as the little _Agonus cataphractus_ or the Seahorse among the Syngnathidae, the body is protected by a complete suit of bony armour; but accompanying these in the littoral region are numerous other species such as the Gobies, and even other species of Syngnathidae which have soft unprotected skins.

Similarly with colour characters: the power of changing the colour so as to harmonize with the ground is obviously beneficial and adaptive, but in each species there is a specific pattern or marking which remains constant throughout life and has nothing to do with protective resemblance, variable or permanent. The red spots of the Plaice are specific and diagnostic, but they confer no advantage over the Dab or the Lemon-dab, in which they are absent, nor can any relation be discovered between these spots and mode of life or habits.

The function of the lateral line organs is still somewhat obscure. The theory that they are sensitive to differences of hydrostatic pressure as the fish moves from one depth to another rests on no foundation, since it has yet to be shown how a change of pressure within the limits of the incompressibility of water can produce a sensation in an organ permeated throughout with water. It is more probable that the organs are affected by vibrations in the water, but we are unable to understand how a difference in the anterior curvature of the lateral line would make a difference in the function in any way related to the difference in conditions of life between Plaice and Dab. There is, however, reason to conclude that the organs, especially on the head, are more important and larger in deeper water, and thus the enlargement of the sensory ca.n.a.ls in the head of the Witch, which lives in deeper water than other species, may be an adaptive character.

Another genus of whose characters I once made a special study is that named _Zeugopterus._ The name was originally given by Gottsche to the largest species _Z. punctatus,_ from the fact that the pelvic fins are united to the ventral, but this character does not occur in other species now included in the genus. There are three species, occurring only in European waters, which form this genus and agree in the following characters. The outline of the body is more nearly rectangular than in other Flat-fishes from the obtuseness of the snout and caudal end, and the somewhat uniform breadth of the body. The surface is rough from the presence of long slender spines on the scales. There is a large perforation in the septum between the gill cavities, but this occurs also in _Arnoglossus megastoma,_ which is placed in another genus. But the generic character of _Zeugopterus,_ which is most important for the present discussion, is the prolongation of the dorsal and ventral fins on to the lower of the body at the base of the tail, the attachments of these accessory portions being transverse to the axis of the body. These fishes have the peculiar habit of adhering to the vertical surfaces of sides of aquaria, even the smooth surfaces of slate or gla.s.s. In nature they are taken occasionally on gravelly or sandy ground, but probably live also among rocks and adhere to them in the same way as to vertical surfaces in captivity. Many years ago (_Journ. Mar. Biol. a.s.sn._, vol. iii 1893-95) I made a careful investigation of the means by which these fishes were able to adhere to a smooth surface, at least in the case of the largest and commonest species _Z. punctatus._ It was observed that so long as the fish was clinging to a vertical surface the posterior parts of the fins were in rhythmical motion, undulations pa.s.sing along them in succession from before backwards, the edge of the body to which they were attached moving with them. The effect of these movements was to pump out water backwards from the s.p.a.ce between the body and the surface it was clinging to, and to cause water to flow into this s.p.a.ce at the anterior edges of the head. The subcaudal flaps were perfectly motionless and tightly pressed between the base of the tail and the surface of support, so that any movement of them was impossible. The question arose, however, whether the tail and these flaps acted as a sucker which aided in the adhesion. The flaps were therefore cut off with scissors--an operation which caused practically no pain or injury to the fish--and it adhered afterwards quite as well as when the fin-flaps were intact. The subcaudal prolongations of the fins are therefore not necessary to the adhesion, nor to the pumping action, of the muscles and fins, which went on as before. It seemed probable, therefore, that the pumping action was itself the cause of the adhesion.

But the difficulty in accepting this conclusion was that there was a distinct though gentle respiratory movement of the jaws and opercula; and if the pumping of the water from beneath the body caused a negative pressure there, and a positive pressure on the outer side of the body, it seemed equally certain that the respiratory movement must force water into the s.p.a.ce beneath the body and so cause a positive pressure there which would tend to force the fish away from the surface with which it was in contact. Examination of the currents of water around the edges of the fish, by means of suspended carmine, showed that water pa.s.sed in at the mouth and out at the lower respiratory orifice, but also into the s.p.a.ce below the body at the upper and lower edges of the head, without pa.s.sing through the respiratory channel. It was thus proved that the rate at which water was pumped out at the sides of the tail was greater than that at which it pa.s.sed in by the respiratory movements, and consequently there a resultant negative pressure beneath the body. By means of a model made of a thin flexible sheet of rubber, at each end of which on one side was fastened a short piece of gla.s.s tube, I was able to imitate the physical action observed in the fish. A long piece of rubber tube was attached to one of the pieces of gla.s.s tube, and brought over the edge of the gla.s.s front of an aquarium. The long rubber tube was set in action as a siphon and the sheet of rubber placed against the gla.s.s. As long as water was running through the siphon the sheet of rubber remained pressed against the gla.s.s and supported. As soon as the current of water was stopped the apparatus fell to the bottom of the tank. In this model water pa.s.sed out from beneath the rubber through the gla.s.s tube attached to the siphon and pa.s.sed in by the opposite gla.s.s tube, and at the sides of it. The latter tube represented the respiratory channel of the fish, and the s.p.a.ce between tube and rubber represented the s.p.a.ces between the head of the fish and the vertical surface to which it clung.

In the fish the marginal fins not only extend to the base of the tail, but are broader at the posterior end than elsewhere, whereas in other Flat-fishes the posterior part of the marginal fins are the narrowest parts. The shape of the fins and the breadth of the body posteriorly, then, are adaptations which have a definite function, that of enabling the fish to adhere to vertical surfaces. But, on the other hand, the extension of the marginal fins in a transverse direction beneath the tail has no use in the process of adhesion, nor has any other use been found for it. It is a generic character, so far as we know, without utility. On the other hand, it is very probable that this subcaudal extension of the fins is merely a result of the posterior extension and enlargement of these fins which has taken place in the evolution of the adaptation. If the Lamarckian explanation of adaptation were true, it would be possible to understand that the constant movements of the fins and muscles by which the adhesion was effected caused a longitudinal growth of the fins in excess of the length actually required, and that this extra growth extended on to the body beneath the tail, although the small flaps on the lower side were not necessary to the new function which the fins performed.

When we consider such cases as this we are led to the conclusion that the usual conception of adaptation is not adequate. We require something more than function or utility to express the difference between the two kinds of characters to be distinguished. For example, the absence of pigmentation from the lower sides of Flat-fishes may have no utility whatever, but we see that it differs from the specific markings of the upper side in the fact that it shows a relation to or correspondence with a difference of external conditions--namely, the incidence of light, while in such a case as the red spots of the Plaice we can discover no such correspondence.

We know that the American artist and naturalist Thayer has shown that the lighter colour of the ventral side of birds and other animals aids greatly in reducing their visibility in their natural surroundings, the diminution in coloration compensating for the diminution in the amount of light falling on the lower side, so that the upper and lower sides reflect approximately the same amount of light, and contrast, which would be otherwise conspicuous, is avoided. But the white lower sides of Flat-fishes are either not visible at all, or, if visible, are very conspicuous, so that the utility of the character is very doubtful.

We may distinguish then between characters which correspond to external conditions, functions, or habits, and those which do not. The word 'adaptation,' which we have hitherto used, does not express satisfactorily the peculiarities of all the characters in the former of these two divisions. If we consider three examples--enlarged hind-legs for jumping as in kangaroo or frog, absence of colour from the lower sides of Flat-fishes, and, thirdly, the finlets on the lower side of _Zeugopterus_--we see that they represent three different kinds of characters, all related to habits or external conditions. We may say that the third kind are correlated with some other character that has a relation to function or external conditions, as the extension of the fins on the under side of _Zeugopterus_ is correlated with the enlargement of the fins, whose function is to cause the adhesion of the fish to a vertical surface.

With regard to the specific characters of the species of _Zeugopterus_ nothing is known of peculiarities in mode of life which would give an importance in the struggle for existence to the concrescence of the pelvic fins with the ventral in _punctatus_, to the absence of this character and the elongation of the first dorsal ray in _unimaculatus_, or to the absence of both characters in _norvegicus_. No use is known for any of the other specific characters, which tend in each case to form a series. Thus in size _norvegicus_ is the smallest, _unimaculatus_ larger, and _punctatus_ largest, the last reaching a of 8-1/2 inches. The subcaudal fin-flaps are developed in _norvegicus_, most in _punctatus_; each has four rays in _norvegicus_ and _unimaculatus_, six in _punctatus_. The shortening and spinulation of the scales are greatest in _punctatus_, least in _norvegicus_. In _punctatus_ there are teeth on the vomer, in _unimaculatus_ none, in _norvegicus_ they are very small.

If we consider fishes in general, we see that there is no evidence of any relation between many of the most important taxonomic characters and function or external conditions. In the seas Elasmobranchs and Teleosteans exist in swarming numbers side by side, but it is impossible to say that one type is more adapted to marine life than the other. There is good reason to believe that bony fishes were evolved from Elasmobranchs in fresh water which was shallow and foul, so that lungs were evolved for breathing air, and that marine bony fishes are descended from fishes with lungs; but no reason has been given for the evolution of bone in place of cartilage or for the various kinds of scales. Professor Houssaye, on the other hand, believes that the number and position of fins is adapted to the shape and velocity of movement of each kind of fish.

If we turn to other groups of animals we find everywhere similar evidence of the distinction between adaptive and non-adaptive characters. Birds are adapted in their whole organization for flight, the structure of the wing, of the sternum, breast muscles, legs, etc., are all co-ordinated for this end. But how do we know that feathers in their origin were connected with flight? It seems equally probable that feathers arose as a mutation in place of scales in a reptile, and the feathers were then adapted for flight. Nothing shows the distinction better than convergent adaptation.

Owls resemble birds of prey in bill and claw and mode of life, yet they are related to insect-eating swifts and goat-suckers and not to eagles and hawks. Swifts and swallows are similar in adaptive characters, but not in those which show relationship. It may be said that the characters believed to show true affinities were originally adaptive, but we do not know this.

Similarly, in reptiles the Chelonia are distinguished by the most extraordinary union of skin-bones and internal skeleton enclosing the body in rigid armour: it may be said that the function of this is protection, that it is adaptation, and can be explained by natural selection, but the adaptation in this case is so indefinite that it is difficult to be convinced of it.

Systematists have always distinguished between adaptive characters and those of taxonomic value--those which show the true affinities--and they are perfectly right: also they have always distrusted and held aloof from theories of evolution which profess to explain all characters by one universal formula. In my opinion, those who, like Weismann, consider all taxonomic characters adaptive, are equally mistaken with Bateson and his followers, who regard all characters as mutational. No system of evolution can be satisfactory unless it recognises that these two kinds of characters are distinct and quite different in their nature. But it may be asked, What objection is there to the theory of natural selection as an explanation of adaptations? The objection is that all the evidence goes to show that the necessary variations only arose under the given conditions, and, further, that the actions of the conditions and the corresponding actions of the organism give rise to stimuli which would produce somatic modifications in the same direction as the permanent modifications which have occurred. My view is, then, that specific characters are usually not adaptations, that other characters of taxonomic value are some adaptive and some unrelated to conditions of life, and that while non-adaptive characters are due to spontaneous blastogenic variations or mutations, adaptive characters are due to the direct influence of stimuli, causing somatic modifications which become hereditary, in other words, to the inheritance of acquired characters. It has become a familiar statement that every individual is the result of its heredity and its environment.

The thesis that I desire to establish is that the heredity of each individual and each type is compounded of variations or changes of two distinct origins, one external and one internal; that is to say, of variations resulting from changes originating in the germ-cells or gametes, and of modifications produced originally in the soma by the action of external stimuli, and subsequently affecting the gametes.

When we study the characters of animals in relation to s.e.x we find that in many cases there are conspicuous organs or characters present in one s.e.x, usually the male, which are absent or rudimentary in the other. The conception of adaptation applies to these also, since we find that characters consist often of weapons such as horns, antlers, and spurs, used in s.e.xual combat, of copulatory or clasping organs such as the pads on a frog's forefeet, of ornamental plumage like the peac.o.c.k's tail serving to charm the female, or of special pouches as in species of pipe-fish and frog for holding the eggs or young. Darwin attempted to explain s.e.xual adaptation by s.e.xual selection. The selective process in this case was supposed to be, not the survival of individuals best adapted to secure food or shelter or to escape from enemies, but the success of those males which were victorious in combat, or which were most attractive to the females, and therefore left the greater number of offspring which inherited their variations. But, as Darwin himself admitted, this theory of selection does not in any way explain the differences between the s.e.xes--in other words, the limitation of the characters or organs to one s.e.x--since there is no reason in the process of selection itself why the peculiarity of a successful male should not be inherited by his female offspring as well as by his male offspring. The real problem, then, is the s.e.x-limited heredity, and we shall consider later whether in this kind of heredity also there are characters of internal as well as external origin, blastogenic as well as somatogenic.

CHAPTER II

Mendelism And The Heredity Of s.e.x

We know that now individuals are developed from single cells which have either been formed by the union of two cells or which develop without such union, and that these reproductive cells are separated from pre-existing organisms: the gametes or gonocytes are separated from the parents and develop into the offspring. The zygote has the power of developing particular structures and characters in the complicated organisation of the adult, and we recognise that the characters are determined by the properties and const.i.tution of the zygote; that is to say, of one or both of the gametes which unite to form the zygote. The distinction between peculiarities or 'characters,' determined in the ovum before development, and modifications due to influences acting on the individual during its development or life, is often obvious enough. A child's health, size, mode of speech, and behaviour may be greatly influenced by feeding, training, and education, but the colour of his or her eyes and hair were determined before birth. A human individual has, we know, a number of congenital or innate characters, by which we mean characters which arise from the const.i.tution of the individual at the time of birth, and not from influences acting on him or her after birth. We have to remember, however, that modifications may be caused during development in the uterus, as, for example, birth-marks on the skin, and these would not be due to peculiarities in the const.i.tution of the ovum. Karl Pearson and other devotees of the cult of Eugenics have been lately impressing on the public by pamphlets, lectures, and addresses the great importance of nature as compared with nurture, maintaining that the latter is powerless to counteract either the good or bad qualities of the former, and that the effects of nurture are not transmitted to the next generation.

We recognise that the characters of varieties of flowers, fruits, and domesticated animals are not to be produced by any particular mode of treatment. We see the various kinds of orchids or carnations in the same greenhouse, of sweet peas and roses in the same garden. We go to a show and see the extraordinary variety of breeds of pigeons, rabbits, or fowls, and we know that these cannot be produced by treating the progeny of individuals of one kind in special ways, but are the progeny of parents of the same various races. If we want fowls of a particular breed we obtain eggs of that breed and hatch them with the certainty born of experience that we shall obtain chickens of that breed which will develop the colour, comb, size, and qualities proper to it. Similarly, in nature we recognise that the 'characters' of species or varieties are not due to circ.u.mstances acting on the individual during its development, but to the properties of the ova or seeds from which the individuals were developed.

Formerly we regarded these congenital or innate characters as derived from the parents or inherited, and heredity was the transmission of const.i.tutional characters from parent to offspring. Now that we fix our attention on the fertilised ovum or the gametes by which it is formed we see that the characters are determined by some properties in the const.i.tution of the gametes. What, then, is heredity? Clearly, it is merely the development in the offspring of the same characters which were present in the ova from which the parents developed. When the characters persist unchanged from generation to generation, we call the process by which they are continued heredity. When new characters appear, _i.e._ new characters determined in the ovum not due to changes in the environment, we call them variations. When a fertilised ovum develops into a new individual, it divides repeatedly to form a very large number of cells united into a single ma.s.s. Gradually the parts of this ma.s.s are differentiated to form the tissues and organs of the body or soma, but some of the cells remain in their original condition and become the reproductive cells which will give rise to the next generation. The reproductive cells also undergo division and increase in number, and when they separate from the new individual and unite in fertilisation they still possess all the determinants of the fertilised ovum from which they are descended. Heredity thus continues from gamete to gamete, not from zygote to soma, and then from soma to gamete.

Modern researches have shown that the nucleus, when the cell divides, a.s.sumes the form of a spindle of fibres, a.s.sociated with which are distinct bodies called chromosomes, that the number of these chromosomes where it can be counted is constant for all individuals of the same species, and that before the gametes are ready for fertilisation two cell-divisions take place, which result in the reduction of the number of chromosomes to half the original number. When two gametes unite, the specific number is restored. Since the male gamete is very small and seems to contribute to the zygote almost nothing except the chromosomes, which carry with them all the characters of the male parent, it seems a necessary conclusion that the chromosomes alone determine the character of the adult. There are, however, facts which point to an opposite conclusion.

Hegner, [Footnote: R. W. Hegner, 'Experiments with Chrysomelid Beetles,'

III., _Biological Bulletin_, vol. xx. 1910-11.] for example, found that in the egg of the beetle _Leptinotarsa_, which is an elongated oval in shape, there is at the posterior end in the superficial cytoplasm a disc-shaped ma.s.s of darkly staining granules, while the fertilised nucleus is in the middle of the egg. When the protoplasm containing these granules was killed with a hot needle, development in some cases took place and an embryo was formed, but the embryo contained no germ cells. Here no injury had been done to the zygote nucleus, but these particular granules and the portion of protoplasm containing them were necessary for the formation of germ cells. In other experiments a large amount of protoplasm at the posterior end of the ovum was killed before the nucleus had begun to segment, and the result was the development of an embryo consisting of the head and part of the thorax, while the rest was wanting. The nucleus segmented and migrated into that part of the superficial cytoplasm which remained alive, and this proceeded to develop that particular part of the embryo to which it would have given rise if the rest of the egg had not been killed. There was no regeneration of the part killed, no formation of a complete embryo. It may be pointed out that segmentation in the insect egg is peculiar. The nuclei multiplied by segmentation migrate into the superficial cytoplasm surrounding the yolk, and then this cytoplasm segments, and each part of the cytoplasm develops into a particular region of the embryo. This, of course, does not prove that the nuclei or their chromosomes do not determine the _characters_ of the parts of the embryo developed, but they show that the parts of the non-nucleated cytoplasm correspond to particular parts of the embryo. The most important object of investigation at the present time is to find the origin of these properties of the chromosomes. We may say, using the word 'determinant' as a convenient term for that which determines the adult characters, that in order to explain the origin of species or the origin of adaptations we must discover the origin of determinants. Mendelism does not throw any direct light on this question, but it certainly has shown how characters may be inherited as separate and independent units. When one difference between two breeds is considered, _e.g._ rose comb and single in fowls, and individuals are crossed, we have the determinant for rose and the determinant for single in the same zygote. The result is that rose develops and single is not apparent. In the next generation rose and single appear, as at the beginning, in separate individuals. When two or three or more differences are studied we find that they are usually inherited separately without connexion with each other, although in some cases they are connected or coupled. The facts of Mendelism are of great interest and importance, but we have to consider the general theory based on them. This theory is that characters are generally separate units which can exist side by side, but do not mingle, and cannot be divided into parts. When an apparently single character shows itself double or treble, it is concluded that it has not been really divided, but consists of two or three units (Castle). Further, although Mendelism in itself shows no evidence of the origin of the characters, it a.s.sumes that they arose as complete units, and one suggestion is that a dominant factor might at some of the divisions in gametegenesis pa.s.s entirely into one daughter cell, and therefore be absent from the other, and thus individuals might be developed in which a dominant character was absent. Bateson in his well-known books, _Mendel's _Principles of Heredity_, 1909, and _Problems of Genetics_, 1913, discusses this question of the origin of the factors which are inherited independently. The difficulty that troubles him is the origin of a dominant character. Naturally, if he persists in regarding the determinant factor as a unit which does not grow nor itself evolve in any way, it is difficult to conceive where it came from. The dominant, according to Bateson, must be due to the presence of something which is absent in the recessive. He gives as an instance the black pigment in the Silky fowl, which is present in the skin and connective tissues. In his own experiments he found this was recessive to the white-skin character of the Brown Leghorn, and he a.s.sumes that the genetic properties of _Gallus bankiva_ with regard to skin pigment are similar to those of the Brown Leghorn. Therefore in order that this character could have arisen in the Silky, the pigment-producing factor _P_ must be added and the inhibiting factor _D_ must drop out or be lost. He says we have no conception of the process by which these events took place. [Footnote: _Problems of Genetics_, p. 85.] Now my experiment in crossing Silky with _bankiva_ shows that no inhibiting factor is present in the latter, so that only one change, not two, was necessary to produce the Silky. Mendelians find it so difficult to conceive of the origin of a new dominant that they even suggest that no such thing ever occurs: what appears as a new character was present from the beginning, but its development was prevented by an inhibiting factor: when this goes into one cell of a division and leaves the other free, the suppressed character appears. This is the principle proposed to get over the difficulty of the origin of a new dominant. All characters are due to factors, and all factors were present in the original ancestor--say Amoeba. Evolution has been merely 'the rejection of various factors from an original complex, and a reshuffling of those that were left.' Professor Lotsy goes so far as to say that difference in species arose solely from crossing, that all domestic animals are of mixed stocks, and that it is easier to believe that a given race was derived from some ancestor of which all trace has been lost than that all races of fowls, for example, arose by variation from a single species, but the evidence that our varieties of pigeons have been derived from _C. livia_, and of fowls from _G. bankiva_, is too strong to be disregarded because it does not agree with theoretical conceptions.

My own experiments in crossing Silky fowls with _Gallus bankiva_ (_P.Z.S._, 1919) show that the recessive is not always pure, that segregation is not in all cases complete. The colour of the _bankiva_ is what is called black-red, these being probably the actual pigments present, mixed in some parts of the plumage, in separate areas in other parts: the Silky is white. There are seven pairs of characters altogether in which the Silky differs from the _bankiva_. Both the pigmented skin of the Silky and the colour in the plumage of the _bankiva_ are dominant, so that all the offspring in _F1_ or the first generation are coloured fowls with pigmented skins. But in later generations I found that with regard to skin pigment there were no pure recessives. Since the heterozygote in _F1_ was deeply pigmented, it is certain that a bird with only a small amount of pigment in its skin was a recessive resulting from incomplete segregation of the pigmented character. The pigment occurred chiefly in the skin of the abdomen and round the eyes, and also in the peritoneum and in the connective tissue of the abdominal wall. It varied in different individuals, but in some, at any rate, was greater in later generations than in the earlier. The condition bred true, as pure recessives do; and when such an impure recessive was mated with a heterozygote with black skin, the offspring were half pigmented and half recessive, with some pigment on the abdomen of the latter.

Still more striking was the incomplete segregation in the plumage colour.

The white of the Silky was recessive, all the birds of the _F1_ generation being fully coloured. In the _F2_ generation there were two recessive white c.o.c.ks which when mature showed slight yellow colour across the loins. These two were mated with coloured hens, and in later generations all the recessives instead of being pure white, like the Silky, had reddish-brown pigment distributed as in pile fowls.

[Ill.u.s.tration: PLATE I. Recessive Pile Fowls]

In the hens (Plate I., fig. 1) it was chiefly confined to the breast and abdomen, and was well developed, not a mere tinge or trace, but a deep coloration, extending on to the dorsal coverts at the lower edge of the folded wings. The back and tail were white. In the c.o.c.ks the colour was much paler, and extended over the dorsal surface of the wings, where it was darker than on the back and loins (Plate I., fig. 2). These pile-coloured fowls when mated together bred true, with individual differences in the offspring.

The pile fowl as recognised and described by fanciers is dominant in colour, not recessive as in the case above described. In fact, a recessive pile does not appear ever to have been mentioned before the publication of the results of my experiment. From the statements of John Douglas in _Wright's Book of Poultry_ (London, 1885), it appears that fanciers knew long ago that the pile could be produced from a female of the black-red Game mated with a white Game-c.o.c.k. It would seem, therefore, that the pile is the heterozygote of black-red and 'dominant' white. Bateson, however (_Principles of Heredity_, 1909, p. 120), writes that the whole problem of the pile is very obscure, and treats it as a case of peculiarity in the genetics of yellow pigments. On p. 102 of the same volume he describes the results of crossing White Leghorn with Indian Game or Brown Leghorn, the _F1_ being substantially white birds with specks of black and brown, though c.o.c.ks have sometimes enough red in the wings to bring them into the category known an pile. To test the matter I have crossed White Leghorns with a pure-bred black-red Game-c.o.c.k, and in the offspring out of eight six were fairly good piles, but with not quite so much red on the back as in typical birds: one was a pile with yellow on the back instead of red, and one was white with irregular specks. Of the hens, four were of pile coloration with breast and abdomen of uniform reddish-brown colour, back, neck, and saddle hackles laced with pale brown, tail white. The other four were white with black and brown specks. Whether these pile heterozygotes will breed true I do not yet know.

These results tend to show that factors are not indivisible units, and segregation is rather the difficulty of chromatin or germ plasm from different race uniting together. It must be remembered that the fertilised ovum which forms one individual gives rise also to dozens or hundreds or thousands or millions of gametes. If a given character is represented by a portion of the chromatin in the original ovum, this has to be divided so many times, and each time to grow to the same condition as before. How can we suppose that the divisions shall be exactly equal or the growth always the same? It is inevitable that irregularities will occur, and if the original chromatin produced a certain character, who shall say what more or less of that chromatin will produce?

In the case of my recessive pile, my interpretation is that when the chromosomes corresponding to two distinct characters such as colour and absence of colour are formed they do not separate from each other completely. Whether the mixture of the chromosomes occurs in every resting stage of the nucleus in the successive generations of the gametocytes, or whether it occurs only in the synapsis stage preceding reduction division, it is not surprising that the colloid substance of the chromosomes should form a more or less complete intermixture, and that the two original chromosomes should not be again separated in the pure condition in which they came into contact. A part, greater or less, of each may be left mixed with the other. This is the probable explanation of the fact that the recessive white plumage has some of the pigment from the dominant form.

Segregation, the repulsion between chromosomes, or chromatin, from gametes of different races may occur in different degrees from complete segregation to complete mixture. When the latter occurs there would be no segregation and the heterozygote would breed true. The most interesting fact is that a given factor in the cases I have described, namely, colour of plumage and pigmentation, of skin in the Jungle fowl and the Silky, is not a permanent and indivisible unit, but is capable of subdivision in any proportion. Bateson has already (in his Address to the Australian meeting of the British a.s.sociation) expressed the same conclusion. He states that although some Mendelians have spoken of genetic factors as permanent and indestructible, he is satisfied that they may occasionally undergo a quant.i.tative disintegration, the results of which he calls subtraction or reduction stages. For example, the Picotee Sweet Pea with its purple edges can be nothing but a condition produced by the factor which ordinarily makes the fully purple flower, quant.i.tatively diminished. He remarks also that these fractional degradations are, it may be inferred, the consequences of irregularities in segregation.

Bateson, however, proceeds to urge that the history of the Sweet Pea belies those ideas of a continuous evolution with which we had formerly to contend. The big varieties came first, the little ones arose later by fractionation, although now the devotees of continuity could arrange them in a graduated series from white to deep purple. Now this may be historically true of the Sweet Pea, but I would point out that once the dogma of the permanent indivisible unit or factor is abandoned, there is nothing in Mendelism inconsistent with the possibility of the gradual increase or decrease of a character in evolution. I do not suggest that the colour and markings of a species or variety were, in all cases, due to external conditions, but if the effect of external stimuli can be inherited, can affect the chromosomes, then the evidence concerning unit factors no longer contradicts the possibility of a character gradually increasing, under the influence of external stimuli acting on the soma from zero to any degree whatever.

s.e.x AND SECONDARY s.e.xUAL CHARACTERS

The mystery of s.e.x is hidden ultimately in the phenomenon of conjugation, that union of two cells which in general seems necessary to the maintenance of life, to be a process of rejuvenation. We know nothing of the nature of this process, or why in general it should produce a reinvigoration of the cell resulting from it. We know little if anything of the relation between the two conjugating cells or gametes, of the real nature of the attraction that causes them to approach each other and ultimately unite together. We have, it is true, some evidence that one cell affects the other by some chemical action, as for instance in the fact that the mobile male gametes of a fern are attracted to a tube containing malic acid, but this may be merely an influence on the direction of movement of the male gamete, while there are cases in which neither cell is actively mobile. What we know in higher animals and plants is that each gamete contains in its nucleus half the number of chromosomes found in the other cells of the parent, and that in the fertilised ovum the chromosomes of both gametes form the new nucleus, in which therefore the original number of chromosomes is restored.

The remarkable fact is that from this fertilised ovum or zygote is developed usually an individual of one s.e.x or the other, male or female, other cases being comparatively exceptional, although each act of fertilisation is the union of the two s.e.xes together. Various attempts have been made to prove that the s.e.x of the organism is determined by conditions affecting it during development subsequent to fertilisation, but now there is good reason to believe that generally the s.e.x of the individual is determined at fertilisation, though as we shall see there is evidence that it may in certain cases be changed at a later stage.

In Mendelian experiments, a heterozygote individual is one arising from gametes containing opposite members of a pair of characters, in other words, from the union of a gamete carrying a dominant with another carrying a recessive. A pure recessive individual is one arising from the union of two gametes both carrying recessives. If a heterozygote is bred with a pure recessive the offspring are half heterozygote and half recessive. The heterozygote individual in typical cases shows the dominant character. In the formation of its gametes when the reduction division of the chromosomes takes place, half of them receive the dominant character, half the recessive. When the division in the gametes of the recessive individual takes place its gametes all contain the recessive character.

Thus, if we indicate the dominant character by _D_ and the recessive by _d_, the const.i.tution of the two individuals is

_Dd_ and _dd_.

The gametes they produce are

_D+d_ and _d+d_,

and the fertilisations are therefore

_Dd_, _Dd_, _dd_, _dd_,

or heterozygote dominants and pure recessives in equal numbers.

It is evident that the reproduction of the s.e.xes is very similar to this.

One of the remarkable facts about s.e.x is that, although the uniting gametes are male and female yet they give rise to males and females in equal numbers. If one s.e.x were a dominant this would be in accordance with Mendelian theory. In accordance with the view that the dominant is something present which is absent in the recessive, the Mendelian theory of s.e.x a.s.sumes that femaleness is dominant, and that maleness is the absence of femaleness, the absence of something which makes the individual female. If we represent the character of femaleness by _F_ and maleness or the recessive by _f_, we have the ordinary s.e.xual union represented by

_Ff_x_ff_;

the gametes will then be

_F_+_f_ and _f_+_f_

and the fertilisations