Hormones and Heredity Part 7

The facts agree with the hormone theory, for it is to be noted that although the development of the s.c.r.o.t.u.m is confined to the males, the 'descent' or dislocation takes place in the foetus, and not at the period of p.u.b.erty. This is in accordance with the fact that the mechanical conditions to which the change is attributed are not related to s.e.xual habits, but to the general habits of life which begin soon after birth.

The development, therefore, may be considered to be related to the presence of a hormone derived from the normal testis, but not to a special quant.i.ty or quality of hormone a.s.sociated with maturity or the functional activity of the organ. In Rodents, however, there is a difference in the organs, not only at maturity, but in every rutting season, at any rate in Muridae such as rats and others. In the rutting season the testes become much larger and descend into the scrotal sacs, at other times of the year being apparently more or less abdominal. In rabbits and hares, which have a much more impulsive progression, the organs seem to be always in the scrotal sacs.

It might be thought that in this case, although the hormone theory of heredity might be applied, there was no reason to suppose that a hormone derived from the testis in the individual development was necessary in order that the hereditary change should take place. If the individual was male and therefore had a testis, this organ would by heredity go through the process of dislocation. But there is the curious fact that when the descent is not normal and complete, in what is called cryptorchidism, the organs are always sterile. The retention of the testes within the abdomen may be regarded as a case of arrested development, like many other abnormalities, but this does not explain why the retained testes should always be sterile, without spermatogenesis. If the inherited or congenital process of dislocation requires the presence of hormones produced by a normal testis, then we can understand why a defective testis does not descend completely, because it does not produce the hormone which is necessary to stimulate the hereditary mechanism to complete dislocation.

It is often stated that in cryptorchidic individuals the s.e.xual instincts and somatic s.e.xual characters are well developed, which would appear contradictory to the above explanation, but according to Ancel and Bouin such individuals in the case of the pig show considerable differences in the secondary signs of s.e.x and in the external genital organs, presenting variations which lie between the normal and the castrated animal.

We have here, then, in the position of the testes in Mammalia a condition which is not in the slightest degree 'adaptive' in the ordinary sense-- that is, fulfilling any special function or utility. The condition must be regarded as distinctly disadvantageous, since the organs are more exposed to injury, and the abdominal wall is weakened, as we know from the risk of scrotal hernia in man. But from the Lamarckian point of view the facts support the conclusion that the condition is the effect of certain mechanical strains, and is of somatic origin, while the correlations here reviewed are entirely unexplained by any theory of mutation or blastogenic origin.

OPPOSING EVIDENCE

We have now to review certain cases which seem to support conclusions contrary to those which we have maintained in the preceding pages, and to consider the evidence which has been published in support of other theories. It must be admitted that the occurrence of male secondary characteristics on one side of the body, and female on the other, is in consistent with the view that the development of such characters is due to the stimulus of a hormone, since the idea of a hormone means something which diffuses by way of the blood-vessels, lymph-vessels, and interstices of the tissues, throughout the body, and the hormone theory of secondary s.e.xual characters a.s.sumes that these characters are potentially present by heredity in both s.e.xes. The occurrence of male somatic characters on one side or in some part of the body and female on the other, usually a.s.sociated with the corresponding gonads, has been termed gynandromorphism, and has long been known in insects. Cases of this condition have been observed, though much more rarely, in Vertebrates. I am not aware of any authentic instances in Mammals, and the supposition that in stags reduction or abnormality of one antler may be the result of removal or injury to the testis of one side, or the opposite, have been completely disproved by experiments in which unilateral castration has been carried out without any effect on the antlers at all. In birds, however, a few cases have been recorded by competent observers with a definiteness of detail which leaves no possibility of doubt. One of the more recent of these is that of a pheasant of the white-ringed Formosan variety, _P. torquatus_, of the Chinese pheasant. [Footnote: C. J. Bond, 'Unilateral Development of Secondary Male Characters in a Pheasant,'

_Journ. of Genetics_, vol. iii., 1914.] On the left side this bird shows the plumage, colour, and the spur of the male; on the right leg there is no spur except the small rudiment normally occurring in the hen. The difference in plumage between the two sides, however, is not complete. The white collar is strictly limited to the left side, but the iridescent blue green of head and neck is present on both sides, though more marked on the left. Only a few male feathers appear in the wing coverts of the left side. The breast feathers are rufous, especially on the left side. The tail coverts show marked male characters, more especially on the left side. In the tail, however, the barred character of the male is not present on one side, absent on the other, but in most of the feathers is confined to one, the _outer_ side of each feather. With regard to the gonads, in this bird a single organ was found on the left side, _i.e._ in the position of the ovary in normal females, and there was no trace of a gonad on the right side. The organ present was small, 3/4 inch long by 1/2 inch broad, and microscopic sections showed in one part actively growing areas of tubular gland structure in some of which bodies like spermatozoa could be detected, while in another were fibrous tissue with degenerating cysts. The latter appear to have been degenerating egg follicles. The author concludes that the organ was originally a functional ovary, and that the ovarian portion had atrophied while a male portion had become functionally active.

Another case in birds was described by Poll [Footnote: _B.B. Ges. Naturf.

Freunde_, Berlin, 1909.] and is mentioned by Doncaster. [Footnote: _Determination of s.e.x_, Cambridge, 1914.] It is that of a Bullfinch which had the male and female plumage sharply separated on the two sides of the body. The right side of the ventral surface was red like a normal male, the left side grey like a normal female. In this case there was a testis on the right side, on the left an ovary as in normal females.

A third case in birds, somewhat different from the two first mentioned, is that of a domestic fowl described by Shattock and Seligmann. [Footnote: _Trans. Pathol. Soc._ (London), vol. 57, Part i., 1906.] It was a bird of the Leghorn breed, two years old, and had the fully developed comb and wattles of the c.o.c.k. Each leg bore a thick blunt spur, nearly an inch in length, but in the Leghorn breed spurs are by no means uncommon in hens of mature age, before they have ceased to lay eggs. In plumage the characters were mainly female. The colour being white could not show s.e.xual differences, the neck hackles were but moderately developed, saddle hackles practically absent, the tail resembled that of the hen. There was a fully developed oviduct on the left side, on the right another less than half the full length. There was also a vas deferens on each side. There was a gonad on each side, that of the right about one-fourth the size of that on the left. In microscopic structure the right gonad resembled a testis consisting entirely of tubuli lined by an epithelium consisting of a single layer of cells. In one part of this organ the tubules were larger than elsewhere, and one of them exhibited spermatogenesis in progress. The left and larger gonad had a quite similar structure, but at its lower end were found two ova enclosed within a follicular epithelium.

With regard to the last case it is to be remarked that though the gonad on the right side was entirely male, there was no unilateral development of male characters. With regard to the other two cases it must be pointed out (1) that the difference between the two somatic s.e.x-characters on the two sides is chiefly a difference of colour, except the difference in the spurs in Bond's pheasant; (2) that the evidence already cited shows that in fowls castration does not prevent the development of the colour and form of the male plumage, nor of the spurs: that in drakes, although castration does not seem to have been carried out on young specimens before the male plumage was developed, when performed on the mature bird it prevents the eclipse, and does not cause the male to resemble the hen.

Castration, then, tends to prove that in Birds the development of the male characters is not so closely dependent on the stimulation of testicular hormone as in Mammals. The characters must therefore be developed by heredity in the soma, which implies that the soma must itself be differentiated in the two s.e.xes. The development must therefore be more in the nature of gametic coupling. It does not follow that the primary s.e.x-character or the somatic characters are exclusive in either s.e.x.

We may suppose that the zygote contains both s.e.xes, one or other of which is dominant, and that dominance of one primary s.e.x involves dominance of the corresponding s.e.xual characters. This does not, however, agree with the result of removal of the ovaries in ducks, for this causes the characters of the male to appear, so that the dominance of the female is not a permanent condition of the soma but is dependent on the ovarian hormone.

In the hermaphrodite individuals mentioned above the difference of dominance is on two sides of the body instead of two different individuals. It may also be remarked here that while it is very difficult to believe that spurs were not due in evolution to the mechanical stimulation of striking with the legs in combat, and while specially enlarged feathers are erected in display, we cannot at present attribute the varied and brilliant _colour_ of male birds to the direct influence of external stimuli.

In Lepidoptera among insects the evidence concerning castration tends to prove that hormones from the gonads play no part at all in the development of somatic s.e.xual characters. Kellog, an American zoologist, in 1905 [Footnote: _Journ. Exper. Zool._ (Baltimore), vol. i., 1905.] described experiments in which he destroyed by means of a hot needle the gonads in silkworm caterpillars (_Bombyx mori_), and found no difference in the s.e.xual characters of the moths reared from such caterpillars. Oudemans had previously obtained the same result in the Gipsy Moth, _Limantria dispar_.

Meisenheimer [Footnote: _Experimentelle Studien zur Soma- und Geschlechtedifferenzierung_. Jena, 1909.] made more extensive experiments on castration of caterpillars in the last-mentioned species, in which the male is dark in colour and has much-feathered antennae, while the female is very pale and has antennae only slightly feathered. In the moths developed from the castrated larvae there was no alteration in the male characters, and in the females the only difference was that some of them were slightly darker than the normal. Meisenheimer and Kopee after him claim to have grafted ovaries into males and testes into females, with the result that the transplanted organs remained alive and grew, and in some cases at least became connected with the genital ducts. Even in these cases the moth when developed showed the original characters of the s.e.x to which belonged the caterpillar from which it came, although it was carrying a gonad of the opposite s.e.x. It will be seen that these results are the direct opposite of those obtained by Steinach on Mammals. We have no evidence that the darker colour of the normal male in this case is adaptive, or due to external stimuli, but the feathering of the antennae is generally believed to const.i.tute a greater development of the olfactory sense organs, and is therefore adaptive, enabling the male to find the female. This is therefore the kind of organ which would be expected to be affected by hormones from the generative organs. It is stated that the s.e.xual instincts were also unaltered, a male containing ovaries instead of testes readily copulating with a normal female.

These results, almost incredible as they appear, are in harmony with the relatively frequent occurrence of gynandromorphism in insects.[Footnote: See Doncaster, _Determination of s.e.x_ (Camb. Univ. Press, 1914), chap.

ix.] One of the most remarkable cases of this is that of an ant (_Myrmica scabrinodis_) the left half of which is male, the right half not merely female, but worker--that is, sterile female, without wing. Cases in Lepidoptera, _e.g. Amphidasys betularia_, have frequently been recorded.

Presumably not only the antennae and markings, but also the genital appendages and the gonads themselves, are male and female on the two sides. On the view that both s.e.xes and the somatic s.e.x-characters of both s.e.xes are present in each zygote, and that the actual s.e.x is due to dominance, we must conclude that the male primary and secondary characters are dominant on one side, and the female on the other, and it is evident that hormones diffusing throughout the body cannot determine the development of somatic s.e.xual characters here. Various attempts have been made to explain gynandromorphism in insects in accordance with the chromosome theory of s.e.x-determination. These are discussed by Doncaster in the volume already cited, but from the point of view of the present work the important question is that concerning the somatic s.e.x-characters.

According to Doncaster it has been found that in some Lepidoptera the different s.e.x-chromosomes occur in the female, not in the male as in other insects. Half the eggs, therefore, contain an X chromosome, and half a Y, while all the sperms contain an X chromosome. Doncaster has seen in _Abraxas grossulariata_ ova with two nuclei both undergoing maturation.

If one of these in reduction expelled a Y chromosome, the other an X, then one would retain an X and the other a Y. Each was fertilised by a sperm, one becoming therefore XX or male and the other XY or female. It may be supposed that as there was only the cytoplasm of one ovum, each nucleus would determine the characters of half the individual developed.

The question remains, therefore, where are the factors of the somatic s.e.x-characters? One suggestion which might be made is that the female characters are present in the _Y_, in this case female producing chromosome, or, if the female characters are merely negative, that the male characters are in the _X_ chromosome, but only show themselves in the h.o.m.ozygous condition, thus:--

FEMALE x MALE XY XX | / | | / | XX YX MALE FEMALE

The male characters in the male, _XX_, would appear because present in two chromosomes, but would be recessive in the female because present only in one chromosome. The validity of this scheme, however, is disproved by the fact that males can transmit the female characters of their race, as in the case mentioned by Doncaster where a male _Nyssia zonaria_ when crossed transmits the wingless character of its own female.

Another, perhaps better, suggestion is that the somatic characters of both s.e.xes are present in each. Then as each somatic cell is descended without segregation from the fertilised ovum, we may suppose that the presence of the s.e.x-chromosomes in the somatic cells themselves in some way determines whether male or female characters shall develop, without the aid of any hormones from the gonads. This theory would be quite compatible with the belief that adaptive somatic s.e.x-characters may be due to external stimulation, for supposing that the hypertrophy or modification is conveyed to the determinants in the gametocytes, and was confined to one s.e.x, _e.g._ the male, then these determinants would be modified in a.s.sociation with the s.e.x-chromosomes of that s.e.x, and thus though after reduction and fertilisation they would be present in the female zygote also, they would not develop in that s.e.x. Thus supposing _M_ to represent a modification acquired in the male and _m_ the absence of the modification, such as the feathered antenna of a moth, and the s.e.x-chromosomes to be _X_ and _Y_, then we should have in the gametocytes--

Male Female

_MM mm_

_XX XY_

Gametes _MX, MX: mXmY_

Zygotes _MmXX male, MmXY female_,

and the character _M_ would only appear in the male because it only develops in a.s.sociation with _XX_ in the somatic cells descended from the male zygote. This would be the result in the first generation in which a somatic modification affected the factors in the chromosomes. In the next generation _m_ in the male would be affected, and the male for the sake of simplicity might be supposed to become _MMXX_. When the female gametes segregated, some would always be _mY_, and some zygotes therefore _MXmY_.

Others might be _MMXY_. On this theory, therefore, there would always be some females heterozygous for the male character.

Geoffrey Smith, one of the many promising young scientific investigators whose careers were cut short in the War, maintained views concerning somatic s.e.x-characters different from that which explains their development as due to a hormone from the testis or ovary. Nussbaum in 1905 [Footnote: 'Ergebuisse der Anat. und Entwicklungsgesch.,' Bd. xv.; _Pflugers Archiv_, Bd. cxxvi, 1909.] had recorded experiments on _Rana fusca_ (which is identical with the British species commonly called _R.

temporaria_) which appeared to prove that in the male frog after castration the annual development of the thumb-pad and the muscles of the fore-leg does not take place, and if these organs have begun to enlarge before castration they atrophy again. When pieces of testis were introduced into the dorsal lymph-sac of a castrated frog the thumb-pads and muscles developed as in a normal frog. Geoffrey Smith and Edgar Schuster [Footnote: _Quart Journ. Mic. Sci_., lvii, 1911-12.] investigated the subject again with results contrary to those of Nussbaum.

Smith and Schuster begin by describing the normal cycle of changes in the testes on the one hand and the thumb-pad on the other. After the discharge of the spermatozoa in March or April the testes are at their smallest size. From this time onwards till August they steadily increase in size, attaining their maximum at the beginning of September. From then till the breeding season no increase in size or alteration of cellular structure occurs, the testes apparently remaining in a state of complete inactivity during this period. With regard to internal development, after the discharge of spermatozoa in the breeding season the spermatogonia divide and proliferate, forming groups of cells known as spermatocysts. In June and July spermatogenesis is active, and from August to October the formation of ripe spermatozoa is completed.

The corresponding changes in the thumb-pads are as follows. Immediately after the breeding season the h.o.r.n.y epidermis of the pad with its deeply pigmented papillae is cast off, and the thumb remains comparatively smooth from April or May until August or September. When the large papillae are shed, smaller papillae remain beneath, and are gradually obliterated by the epidermis growing up between them. The epidermis is therefore growing while the spermatogenesis is taking place. In August and September the epidermic papillae begin to be obvious, and from this time till February a continuous increase in the papillae and their pigmentation occur. Geoffrey Smith argues that the development of this somatic character occurs while the testes are inactive and unchanged. Considering that the testes throughout the winter months are crammed with spermatozoa, which must require some nourishment, and which may be giving off a hormone all the time, the argument has very little weight. Smith and Schuster found that ovariotomy, with or without subsequent implantation of testes or injection of testis extract, had no effect in causing the thumb of the female to a.s.sume any male characters.

Castration during the breeding season causes the external pigmented layer with its papillae to be cast off very soon--that is to say, it has the same effect as the normal discharge of the spermatozoa. Smith and Schuster found that castration at other seasons caused the pad to remain in the condition in which it was at the time, that there was no reduction or absorption as Nussbaum and Meisenheimer found, and that allo-transplantation of testes--that is, the introduction of testes from other frogs either into the dorsal lymph-sacs or into the abdominal cavity--or the injection of testis extract, had no effect in causing growth or development of the thumb-pad.

There seems to be one defect in the papers of both Nussbaum and Smith and Schuster--namely, that neither of them mentions or apparently appreciates the fact that the thumb-pads, apart from the dermal glands, consist of h.o.r.n.y epidermis developed from the living epidermis beneath. The h.o.r.n.y layer is not shown clearly in the figures of Smith and Schuster. It seems impossible that the h.o.r.n.y layer or its papillae could atrophy in consequence of castration, or be absorbed. The h.o.r.n.y part of the frog's thumb-pad is comparable with the h.o.r.n.y sheath of the horns in the mammalian p.r.o.ng-buck (_Antilocapra_) which are shed after the breeding season and annually redeveloped. Meisenheimer claims that he produced development of papillae on the thumb-pad, not only by implantation of pieces of testis, but also by implantation of pieces of ovary. This seems so very improbable that it suggests a doubt whether the same investigator was not mistaken with regard to the results of his experiments in transplanting gonads in Moths.

Smith and Schuster conclude that the normal development of the thumb-pad depends on the presence of normal testes, but that there is no sufficient evidence that the effect is due to a hormone derived from the testis. It is equally probable, according to Smith, that the testicular cells take up some substance or substances from the blood, thus altering the composition of the latter and perhaps stimulating the production of these substances in some other organ of the body. These substances may be provisionally called s.e.xual formative substances. Smith's theory therefore is that the action of the testes in metabolism is rather to take something from the blood than to add something to it, and that it is this subtractive effect which influences the development of somatic s.e.xual organs.

Geoffrey Smith in fact, in the paper above considered, attempts to apply to the frog the views he put forward [Footnote: _Fauna und Flora des Golfes van Neapel_, 29 Monographie Rhizocephala.] in relation to the effect of the parasite _Sacculina_ on the s.e.xual organs of crabs. The species in which he made the most complete investigation of the influence of the parasite was _Inachus scorpio_ (or _dorsettensis_). Figures showing the changes in the abdomen produced by the presence of _Sacculina_ are given in Doncaster's _Determination of s.e.x_, Pl. xv. _Sacculina_ is one of the Cirripedia, and therefore allied to the Barnacles. It penetrates into the crab in its larval stage, and pa.s.ses entirely into the crab's body, where it develops a system of branching root-like processes. When mature the body of the _Sacculina_ containing its generative organs forms a projection at the base of the abdomen of the crab on its ventral surface, and after this is formed the crab does not moult. Crabs so affected do not show the usual somatic s.e.xual characters, and at one time it was supposed that only females were attacked. It is now known that both s.e.xes of the host may be infected by the parasite, but the presence of the latter causes suppression of the somatic s.e.x-differences. The entry of the parasite is effected when the crab is young and small, before the somatic s.e.x-characters are fully developed. The gonads are not actually penetrated, at least in some cases, by the fibrous processes of the parasite, but nevertheless they are atrophied and almost disappear. In _Inachus_ the abdomen of the normal male is very narrow and has no appendages except two pairs of copulatory styles. The abdomen of the female is very broad, and has four pairs of biramous appendages covered with hairs, the normal function of which is to carry the eggs. The effect of the parasite in the male is that the abdomen is broader, the copulatory styles reduced, and biramous hairy appendages are developed similar to those of the female, but smaller. In the female the abdomen remains broad, but the appendages are much smaller than in the normal female, about equal in size to those of the 'sacculinised' male. Smith interpreted the alteration in the male as a development of female secondary characters, but it is obvious from the condition in Macrura or tailed Decapods, like the lobster or crayfish, that the abdomen or tail of the male originally carried appendages similar to those of the female, and that the male character is a loss of these appendages. The absence of the male character therefore necessarily involves a development of these appendages, and there is not much more reason for saying that the male under the influence of the parasite develops female characters, than for saying that the male character is absent. There is no evidence in the facts concerning parasitic castration for Geoffrey Smith's conclusion that the female characters are latent in the male, but the male characters not latent in the female: both return to a condition in which they resemble each other, and the primitive form from which they were differentiated.

By his studies of parasitic castration Geoffrey Smith was led to formulate a theory for the explanation of somatic s.e.x-characters different from that of hormones. He found that in the normal female crab the blood contained fatty substances which were absorbed by the ovaries for the production of the yolk of the ova. When _Sacculina_ is present these substances are absorbed by the parasite; the ovary is deprived of them, and therefore atrophies. In the male the parasite requires similar substances, and its demand on the blood of the host stimulates the secretion of such substances, so that the whole metabolism is altered and a.s.similated to that of the female. It is this physiological change which causes the development of female secondary characters. He describes this change as the production of a hermaphrodite s.e.xual formative substance, on the ground that in at least one case eggs were found in the testis of a male _Inachus_ which had been the host of a _Sacculina_, but had recovered. It must however be noted that the _Sacculina_ itself is hermaphrodite, with ovaries much larger than the testes. It is possible that while the parasite prevents the development of testis or ovary in the host, it gives up to the body of the host a hormone from its own ovaries which tends to develop the female secondary characters: for the parasite is itself a Crustacean, and therefore the hormone from its ovaries would not be of too different a nature to act upon the tissues of the host.

The observation of Geoffrey Smith that eggs may occur in the testis of a crab after recovery from the parasite appears of more importance than his peculiar theoretical suggestions, for it tends to show that s.e.x is not always unalterably fixed at fertilisation. In this case the influence of a parasite predominantly female would seem to be the real cause of the development of eggs in the testis of the host. Geoffrey Smith does not discuss the origin of the somatic s.e.xual characters in evolution, or attempt to show how his theories of s.e.xual formative substance, and of the influence of the gonads by subtraction rather than addition, would bear upon the problem.

CHAPTER VI

Origin Of Non-s.e.xual Characters: The Phenomena Of Mutation

According to the theory here advocated, modifications produced by external stimuli in the soma will also be inherited in some slight degree in each generation when they have no relation to s.e.x or reproduction. In this case the habits and the stimuli which they involve will be common to both s.e.xes, and the hormones given off by the hypertrophied tissues will act upon the corresponding determinants in the gametocytes. The modifications thus produced will therefore be related to habits, and the theory will include all adaptations of structure to function, but other characters may also be included which are the result of stimuli and yet have no function or utility.

The majority of evolutionists in recent years have taught that influences exerted through the soma have no effect on the determinants in the chromosomes of the gametes, that all hereditary variations are gametogenic and none somatogenic. Mendelians believe that evolution has been due to the appearance of characters or factors of the same kind as those which distinguish varieties in cultivated organisms, and which are the subject of their experiments, but they have found a difficulty, as already mentioned in Chapter II, in forming any idea of the origin of a new dominant character. A recessive character is the absence of some positive character, and if in the cell-divisions of gametogenesis the factor for the positive character pa.s.ses wholly into one cell, the other will be without it, will not 'carry' that factor. If such a gamete is fertilised by a normal gamete the organism developed from the zygote will be heterozygous, and segregation will take place in its gametes between the chromosome carrying the factor and the other without it, so that there will now be many gametes dest.i.tute of the factor in question. When two such gametes unite in fertilisation the resulting organism will be a h.o.m.ozygous recessive, and the corresponding character will be absent. In this way we can conceive the origin of albino individuals from a coloured race, supposing the colour was due to a single factor.

In Bateson's opinion the origin of a new dominant is a much more difficult problem. In 1913 he discussed the question in his Silliman Lectures.

[Footnote: _Problems of Genetics_, Oxford Univ. Press, 1913.] He considers the difficulty is equally hopeless whether we imagine the dominants to be due to some change internal to the organism or to the a.s.sumption of something from without. Accounts of the origin of new dominants under observation in plants usually prove to be open to the suspicion that the plant was introduced by some accident, or that it arose from a previous cross, or that it was due to the meeting of complementary factors. In medical literature, however, there are numerous records of the spontaneous origin of various abnormalities which behave as dominants, such as brachydactyly, and Bateson considers the authenticity of some of these to be beyond doubt. He concludes that it is impossible in the present state of knowledge to offer any explanation of the origin of dominant characters. In a note, however, he suggests the possibility that there are no such things as new dominants. Factors have been discovered which simply inhibit or prevent the development of other characters. For example, the white of the plumage in the White Leghorn fowl is due to an inhibiting factor which prevents the development of the colour factor which is also present. Withdraw the dominant inhibiting factor, and the colour shows itself. This is shown by crossing the dominant white with a recessive white, when some birds of the F(2) generation are coloured.[Footnote: Bateson, _Principles of Heredity_, p. 104.] Similarly, brachydactyly in man may be due to the loss of an inhibiting factor which prevents it appearing in normal persons. It is evident, however, that it is difficult to apply this suggestion to all cases. For example, the White Leghorn fowl must have descended from a coloured form, probably from the wild species _Gallus bankiva_. If Bateson's suggestion were valid we should have to suppose that the loss of the factor for colour caused the dominant white to appear, and then when this is withdrawn colour appears again, so that the colour factors and the inhibiting factors must lie over one another in a kind of stratified alternation. And then how should we account for the recessive white?

In his Presidential Address to the meeting of the British a.s.sociation in Australia, 1914, Bateson explains his suggestion somewhat more fully with a command of language which is scarcely less remarkable than the subject matter. The more true-breeding forms are studied the more difficult it is to understand how they can vary, how a variation can arise. When two forms of _Antirrhinum_ are crossed there is in the second generation such a profusion of different combinations of the factors in the two grandparents, that Lotsy has suggested that all variations may be due to crossing. Bateson does not agree with this. He believes that genetic factors are not permanent and indestructible, but may undergo quant.i.tative disintegration or fractionation, producing subtraction or reduction stages, as in the Picotee Sweet Pea, or the Dutch Rabbit. Also variation may take place by loss of factors as in the origin of the white Sweet Pea from the coloured. But regarding a factor as something which, although it may be divided, neither grows nor dwindles, neither develops nor decays, the Mendelian cannot conceive its beginning any more than we can conceive the creation of something out of nothing. Bateson asks us to consider therefore whether all the divers types of life may not have been produced by the gradual unpacking of an original complexity in the primordial, probably unicellular forms, from which existing species and varieties have descended. Such a suggestion in the present writer's opinion is in one sense a truism and in another an absurdity. That the potentiality of all the characters of all the forms that have existed, pterodactyls, dinosaurs, b.u.t.terflies, birds, etc. etc., including the characters of all the varieties of the human race and of human individuals, must have been present in the primordial ancestral protoplasm, is a truism, for if the possibility of such evolution did not exist, evolution would not have taken place. But that every distinct hereditary character of man was actually present as a Mendelian factor in the ancestral _Amoeba_, and that man is merely a group of the whole complex of characters allowed to produce real effects by the removal of a host of inhibiting factors, is incredible. The truth is that biological processes are not within our powers of conception as those of physics and chemistry are, and Bateson's hypothesis is nothing but the old theory of preformation in ontogeny. Just as the old embryologists conceived the adult individual to be contained with all its organs to the most minute details within the protoplasm of the fertilised ovum or one of the gametes, so the modern Mendelian, because he is unable to conceive or to obtain the evidence of the gradual development of a hereditary factor, conceives all the hereditary factors of the whole animal kingdom packed in infinite complexity within the protoplasm of the primordial living cells. That man is complex and _Amoeba_ simple is merely a delusion; the truth according to Mendelism is that man is merely a fragment of the complexity of the original _Amoeba_.

Mendelism studies especially the heredity of characters, and only incidentally deals with recorded instances of the appearance of new forms, such as the origin of a salmon-coloured variety of _Primula_ from a crimson variety. The occurrence of new characters, or mutations as they are called, has been specially studied by other investigators, and I propose briefly to consider the two most important examples of such research, namely, that by Professor T. H. Morgan, which deals with the American fruit-fly _Drosophla_, and the other which concerns the mutations of the genus of plants OEnothera, exemplified by our well-known Evening Primrose.

Professor T. H. Morgan informs us [Footnote: _A Critique of the Theory of Evolution_ (Oxford Univ. Press, 1916), p. 60] that within five or six years in laboratory cultures of the fruit-fly, _Drosophila ampelophila_, arose over a hundred and twenty-five new types whose origin was completely known. The first of these which he mentions is that of eye colour, differing in the two s.e.xes, in the female dark eosin, in the male yellowish eosin. Another mutation was a change of the third segment of the thorax into a segment similar to the second. Normally the third segment bears minute appendages which are the vestiges of the second pair of wings; in the mutant the wings of the third segment are true wings though imperfectly developed. A factor has also occurred which causes duplication of the legs. Another mutation is loss of the eyes, but in different individuals pieces of the eye may be present, and the variation is so wide that it ranges from eyes which until carefully examined appear normal, to the total absence of eyes. Wingless flies also arose by a single mutation.

These were found on mating with normal specimens to be all recessive characters, thus agreeing with Bateson's views. The next one described is dominant. A single male appeared with a narrow vertical red bar instead of the broad red normal eye. When this male was bred with normal females all the eyes of the offspring were narrower than the normal eye, though not so narrow as in the abnormal male parent. It may be pointed out that this is scarcely a sufficient proof of dominance. If the mutation were due to the loss of one factor affecting the eye, the heterozygote carrying the normal factor from the mother only might very well develop a somewhat imperfect eye.

Morgan arranges the numerous mutations observed in _Drosophila_ in four groups, corresponding in his opinion to the four pairs of chromosomes occurring in the cells of the insect. After the meiotic or reduction divisions each gamete of course contains in its nucleus four single chromosomes. One of the four pairs consists of the s.e.x-chromosomes. All the factors of one group are contained in one chromosome, and it is found in experiments that the members of each group tend to be inherited together--that is to say, if two or more enter a cross together, in other words, if a specimen possessing two or more mutations is crossed with another in which they are absent, they tend to segregate as though they were a single factor. This fact agrees with the hypothesis that the factors in such a case are contained in a single chromosome which segregates from the fellow of its pair in the reduction divisions.

Exceptions may occur, however, and these are explained by what is called 'crossing over.' When one chromosome of a pair, instead of being parallel to the other in the gametocyte, crosses it at a point of contact, then when the chromosomes separate, part of one chromosome remains connected with the part of the other on the same side and the two parts separate as a new chromosome, so that two factors originally in the same chromosome may thus come to lie in different chromosomes. In consequence of this, two or more factors which are usually 'coupled' or inherited together may come to appear in different individuals.

Morgan emphasises the statement that a factor does not affect only one particular organ or part of the body. It may have a chief effect in one kind of organ, _e.g._ the wings or eyes, but usually affects several parts of the body. Thus the factor that causes rudimentary wings also produces sterility in females, general loss of vigour, and short hind legs.