_Ff_ and _ff_,

or males and females in equal numbers, as they are, at least approximately, in fact.

The close agreement of this theory with what actually happens is certainly important and suggests that it contains some truth. But it cannot be said to be a satisfactory explanation. It ignores the question of the nature of s.e.x. According to the theory the female character is entirely wanting in the male. But what is s.e.x but the difference between ovum and spermatozoon, between megagamete and microgamete? The theory then a.s.serts that an individual developed from a cell formed by the union of male and female gametes is entirely incapable of producing female gametes again.

Every zygote after conjugation or fertilisation may be said to be bis.e.xual or hermaphrodite. How comes it then that the female quality entirely disappears? Whether the gametocytes are distinguishable at an early stage in the segmentation of the ovum, or only at a later stage of development, we know that the gametes ultimately formed have descended by a series of cell-divisions from the fertilised ovum or zygote cell from which development commenced. If segregation takes place at the reduction divisions we might suppose that half the gametes formed are sperms and half are ova, and that in the male the latter do not survive but perish and disappear. But in this case it would be the whole of the chromosomes coming from the original female gamete which would disappear, and the spermatozoon would be incapable of transmitting characters derived from the female parent of the individual in which the spermatozoa were formed.

An individual could never inherit character from its paternal grandmother.

This, of course, is contrary to the results of ordinary Mendelian experiments, for characters are inherited equally from individuals of either s.e.x, except secondary s.e.xual characters and s.e.x-linked characters which we shall consider later.

Similarly, if we suppose that segregation of ovum and sperm occurs in the female, the sperms must disappear and the ovum would contain no factors derived from the male parent. But the theory supposes that the segregation of male and female does occur in the female, that half the ova are female and half are male. What meaning are we to attach to the words 'male ovum'

or even 'male producing ovum'? It is a fundamental principle of Mendelism that the soma does not influence the gametocytes or gametes; we have therefore only to consider the s.e.x of the gametes themselves, derived from a zygote which is formed by the union of two s.e.xes. The quality of maleness consists only in the size, form, and mobility of the sperm in the higher animals and of the microgamete in other cases. In what sense then, can an ovum be male? It may perhaps be said that though it is itself female, it has some property or factor which when united with a sperm causes the zygote to be capable of producing only sperms, and conversely the female ovum has a quality which causes the zygote to produce only ova.

But since these qualities segregate in the reduction divisions, how is it that the male quality in the _f_ ovum does not make it a sperm? We are asked to conceive a quality, or the absence of a factor, in an ovum which is incapable of causing that ovum to be a sperm, but which, when segregated in the gametes descended from that ovum, causes them all to be sperms. It is impossible to conceive a single quality or factor which at different times produces directly opposite effects. The Mendelian theory is merely a theory in words, which have an apparent relation to the facts, but which when examined do not correspond to any real conceptions.

However, we have to consider a number of remarkable facts concerning the relation of chromosomes to s.e.x. In the ants, bees, and wasps the unfertilised ovum always develops into a male, the fertilised into a female. The chromosomes of the ovum undergo reduction in the usual way, and are only half the number of those present in the nucleus before reduction. We may call this reduced number _N_ and the full number _2N_.

The ova developing by parthenogenesis and giving rise to males segment in the usual way, and all the cells both of soma and gametocytes contain only _N_ chromosomes. In the maturation divisions reduction does not occur, _N_ chromosomes pa.s.sing to one gamete, none to the other, and the latter perishes so that the sperms all contain _N_ chromosomes. When fertilisation occurs the zygote therefore contains _2N_ chromosomes and becomes female. Here then we have no segregation of _Fxf_ in the ova. The difference of s.e.x merely corresponds to duplex and simplex conditions of nucleus, but it is curious that the simplex condition in the gametes occurs in both ova and sperms.

In Daphnia and Rotifers the facts are different. Parthenogenesis occurs when food supply is plentiful and temperature high. In this case reduction of the chromosomes does not occur at all, the eggs develop with _2N_ chromosomes and all develop into females. Under unfavourable conditions reduction or meiosis occurs, and two kinds of eggs larger and smaller are formed, both with _N_ chromosomes. The larger only develops when fertilised and give rise to females with _2N_ chromosomes. The smaller eggs develop without fertilisation, by parthenogenesis, and become males.

Here then we have three kinds of gametes, large eggs, small eggs, and sperms, each with the same number of chromosomes. It is not the mere number then which makes the difference, but we find a segregation in the ova into what may for convenience be called female ova and male ova.

In Aphidae or plant lice a third condition is found. Here again parthenogenesis continues for generation after generation so long as conditions are favourable, _i.e._ in summer, and the eggs are in the same condition as in Daphnia, etc., that is to say, reduction does not occur, and the number of chromosomes is 2_N_. Under unfavourable conditions males are developed as well as females by parthenogenesis, but the males arise from eggs which undergo partial reduction of chromosomes, only one or two being separated instead of half the whole number. The number then in an egg which develops into a male is 2_N_-1, while other eggs undergo complete reduction and then have _N_ chromosomes. The latter, however, do not develop until they have been fertilised. In the males, when mature, reduction takes place in the gametes, so that two kinds of sperms are formed, those with _N_ chromosomes and those with _N_-l chromosomes. The latter degenerate and die, the former fertilise the ova, and the fertilised ova develop only into females. The chief difference in this case then is that the reduction in the male to the _N_ or simplex condition takes place in two stages, one in the parthenogenetic ovum, one in the gametes of the mature male. In Hymenoptera and in Daphnia, etc., the whole reduction takes place in the parthenogenetic ovum, and in the mature male, though reduction divisions occur, no separation of chromosomes takes place: at the first division one cell is formed with _N_ chromosomes and one with none, and the latter perishes.

In many insects and other Arthropods which are not parthenogenetic the male has been found to possess fewer chromosomes than the female. The female forms, as in the above cases of parthenogenesis, only gametes of one kind each with _N_ chromosomes, but the male forms gametes of two sorts, one with N chromosomes, the other with _N_-l or _N_-2 chromosomes.

On fertilisation two kinds of zygotes are formed, female-producing eggs with 2_N_ chromosomes, and male-producing eggs with 2_N_-1 or 2_N_-2 chromosomes. There is also evidence that in some cases, _e.g._ the sea-urchin, the female is heterozygous, forming gametes, some with _N_ and some with _N_+ chromosomes, while the male gametes are all _N_.

Fertilisation then produces male-producing eggs with 2_N_ chromosomes, female-producing with 2_N_+.

Such is the summary given by Castle in 1912. [Footnote: _Heredity and Eugenics_, by Castle and Others. University of Chicago Press, 1912.] It will be seen that he treats the differences as purely quant.i.tative, mere differences in the number of the chromosomes. Professor E. B. Wilson, however, who had contributed largely by his own researches to our knowledge of s.e.x from the cytological point of view, had already published, in 1910, [Footnote: '_The Determination of s.e.x_,' _Science Progress_, April 1910.] a very instructive _resume_ of the facts observed up to that time. The important fact which is generally true for insects, according to Wilson, is that there is a special chromosome or chromosomes which can be distinguished from the others, and which is or are related to s.e.x differentiation. This chromosome, to speak of it for convenience in the singular, has been variously named by different investigators. Wilson called it the 'X chromosome,' McCluny the 'accessory chromosome,'

Montgomery the 'hetero-chromosome,' while the names 'heterotropic chromosome' and idiochromosome have also been used. For the purpose of the present discussion we may conveniently name it the s.e.x-chromosome. It is often distinguished by its larger size and different shape. Wilson describes the following different cases:--

(1) The s.e.x-chromosome in the male gametocytes is single and fails to divide with the others, but pa.s.ses undivided to one pole. This may occur in the first reduction division (Orthoptera, Coleoptera, Diptera) or in the second (many Hemiptera). But it is difficult to understand what is meant by 'fails to divide.' In one of the reduction divisions all the chromosomes divide as in ordinary or h.o.m.otypic nucleus division, but in the other the chromosomes simply separate into two equal groups without division. If there are an odd number of chromosomes, 2_N_-1, in all the gametocytes of the male, as stated in most accounts of the subject, then if one chromosome fails to divide in the h.o.m.otypic division, we shall have 2_N_-2 in one spermatocyte and 2_N_-1 in the other. Then when the heterotypic division takes place and the number of chromosomes is halved, we shall have two spermatocytes with _N_-1 chromosomes from one of the first spermatocytes and one with _N_ and one with _N_-1 from the other.

Thus there will be three spermatozoa with _N_-1 chromosomes and one with _N_ chromosomes, whereas we are supposed to find equal numbers with _N_ and _N_-1 chromosomes. It is evident that what Dr. Wilson means is that the s.e.x-chromosome is unpaired, and that although it divides like the others in the h.o.m.otypic division, in the heterotypic division it has no mate and so pa.s.ses with half the number of chromosomes to one pole of the division spindle, while the other group of chromosomes has no s.e.x-chromosome. Examples of this are the genera _Pyrrhocoris_ and _Protenor_ (Hemiptera) _Brachystola_ and many other Acrididae, _Anasa, Euthoetha, Narnia, Anax_. In a second cla.s.s of cases the s.e.x-chromosome is double, consisting of two components which pa.s.s together to one pole.

Examples of this are _Syromaster, Phylloxera, Agalena_. In a third cla.s.s the s.e.x-chromosome is accompanied by a fellow which is usually smaller, and the two separate at the differential division. The sizes of the two differ in different degrees, from cases as in many Coleoptera and Diptera in which the smaller chromosome is very minute, to those (_Benacus, Mineus_) in which it is almost as large as its fellow, and others (_Nezara, Oncopeltus_) in which the two are equal in size. Again, there are cases in which one s.e.x-chromosome, say _X_, is double, triple, or even quadruple, while the other, say _Y_, is single. In all these cases there are two _X_ chromosomes in the oocytes (and somatic cells) of the female, and after reduction the female gametes or unfertilised ova are all alike, having a single _X_ chromosome or group. On fertilisation half the zygotes have _XX_ and half _XY_, whether _Y_ is absence of a s.e.x-chromosome, or one of the other _Y_ forms above mentioned. The s.e.x is thus determined by the male gamete, the _X_ chromosome united with that of the female gamete producing female individuals, while the _Y_ united with _X_ produces male individuals.

Professor T. H. Morgan has made numerous observations and experiments on a single culture of the fruit-fly, _Drosophila ampelophila_, bred in bottles in the laboratory for five or six years. He has not only studied the chromosomes in the gametes of this fly, and made Mendelian crosses with it, but has obtained numerous mutations, so that his work is a very important contribution to the mutation doctrine. Drosophila in the hands of Professor Morgan and his students and colleagues has thus become as cla.s.sical a type as Oenothera in those of the botanical mutationists.

Different branches of Morgan's work are discussed elsewhere in this volume, but here we are concerned only with its bearing on the question of the determination of s.e.x. He describes [Footnote: _A Critique of the Theory of Evolution_. Princeton University Press and Oxford University Press, 1916.] the chromosomes of Drosophila as consisting in the diploid condition of four pairs, that is to say, pairs which separate in the reduction division so that the gamete contains four single chromosomes, one of each pair. In two of these pairs the chromosomes are elongated and shaped like boomerangs, in the third they are small, round granules, and the fourth pair are the s.e.x-chromosomes: in the female these last are straight rods, in the male one is straight as in the female, the other is bent. The straight ones are called the X chromosomes, the bent one the Y chromosome. The fertilisations are thus XX which develops into a female fly, and XY which develops into a male. Drosophila therefore is an example of one of the cases described by Wilson.

Dr. Wilson (_loc. cit._) discusses the question of how we are to interpret these facts, in particular, the fact that the X chromosome in fertilisation gives rise to females. He remarks that the X chromosome must be a male-determining factor since in many cases it is the only s.e.x-chromosome in the males, yet its introduction into the egg establishes the _female_ condition. This is the same difficulty which I pointed out above in connection with the Mendelian theory that the female was heterozygous and the male h.o.m.ozygous for s.e.x. Dr. Wilson points out that in the bee, where fertilised eggs develop into females and unfertilised into males, we should have to a.s.sume that the _X_ chromosome in the female gamete is a female determiner which meets a recessive male determiner in the _X_ chromosomes of the sperm. When reduction occurs, the _X_[female]

must be eliminated since the reduced egg develops always into a male. But on fertilisation, since the fertilised egg develops into a female, a dominant _X_[female] must come from the sperm, so that our first a.s.sumption contradicts itself.

Dr. Wilson, T. H. Morgan, and Richard Hartwig have therefore suggested that the s.e.x-difference as regards gametes is not a qualitative but a quant.i.tative one. In certain cases there is no evident quant.i.tative difference of chromatin as a whole, but there may in all cases be a difference in the quant.i.ty of special s.e.x-chromatin contained in the _X_ element. The theory put forward by Wilson then is that a single _X_ element means _per se_ the male condition, while the addition of a second element of the same kind produces the female condition. Such a theory might apply even to cases where no s.e.x-chromosomes can be distinguished by the eye: the ova, in such cases (probably the majority), might also have a double dose of s.e.x-chromatin, the males a single dose. This theory, however, is still open to the objection that the female gametes before fertilisation, and half the male gametes, have the half quant.i.ty of s.e.x-chromatin which by hypothesis determines the male condition, so that here again we have the male condition as something which is distinct from the characteristics of the spermatozoon. But if this is the case, what is the male condition? The parthenogenetic ovum of the bee is male, and yet it is an ovum capable only of producing spermatozoa. If the single X chromosomes is the cause of the development of spermatozoa in the male bee, why does it not produce spermatozoa in the gametes of the female bee, since when reduction takes place all these gametes have a single X chromosome?

In biology, as in every other science, we must admit facts even when we cannot explain them. The facts of what we call gravitation are obvious, and any attempt to disregard them would result in disaster, yet no satisfactory explanation of gravitation has yet been discovered: many theories have been suggested, but no theory has yet been proved to be true. In the same way it may be necessary to admit that two X chromosomes result in the development of a female, and one X, or XY chromosomes result in the development of a male. But Mendelians have omitted to consider what is meant by male and female. The soma with its male and female somatic characters has nothing to do with the question, since somatic s.e.x-differences may be altogether wanting, and moreover, the essential male character, the formation of spermatozoa, is by the Mendelian hypothesis due to descent of the male gametes from the original fertilised or unfertilised _ovum_. The Mendelian theory therefore is that when an ovum has two X s.e.x-chromosomes it can only after a number of cell-divisions, at the following reduction division, give rise to ova, while an ovum containing one X s.e.x-chromosome, or two different, XY, chromosomes, at the next reduction division gives rise to spermatozoa. The X s.e.x-chromosome is not in itself either female or male, since, as we have seen, either ovum or spermatozoon may contain a single X chromosome. The ovum then with one X chromosome or one X and one Y changes its s.e.x at the next reduction division and becomes male. In parthenogenetic ova this happens without conjugation with a spermatozoon at all: in other cases, since the zygote is compounded of spermatozoon and ovum, we can only say that in the XX zygote, the ovum developing only ova, the female is dominant, in the X or XY zygote developing only spermatozoa the male is dominant. Hermaphrodite animals, as has been pointed out by Correns and Wilson, cannot be brought under this scheme at all. In the earthworms, for instance, we have, in every individual developed from a zygote, ova and spermatozoa developing in different gonads in different parts of the body.

The differentiation here, therefore, must occur in some cell-division preceding the reduction divisions. Every zygote must have the same composition, and yet give rise to two s.e.xes in the same individual.

Further light on the s.e.x problem, as in many other problems in biology, can only be obtained by more knowledge of the physical and chemical processes which take place in the chromosomes and in the relations of these structures to the rest of the cell. The recent advances in cytology, remarkable as they are, consist almost entirely of observations of microscopic structure. They may be said to reveal the statics of the cell rather than its dynamics. Cytology is in fact a branch of anatomy, and in the anatomy of the cell we have made some progress, but our knowledge of the physiology of the cell is still infinitesimal. The nucleus, and especially the chromosomes, are supposed in some unknown way to influence or govern the metabolism of the cytoplasm. From this point of view the hypothesis mentioned above that the s.e.x-difference in the gametes is not qualitative but quant.i.tative is probably nearer to the truth. Geddes and Thomson and others have maintained that the s.e.x-difference is one of metabolism, the ovum being more anabolic, the sperm more katabolic. A double quant.i.ty of special chromatin may be the cause of the greater anabolism of the ovum. In that case the difficulty indicated in a previous part of this chapter, that the ovum after reduction resembles the sperm in having only one X chromosome, may be explained by the fact that the growth of the ovum and its acc.u.mulation of yolk substances has been already accomplished under the influence of the two chromosomes before reduction.

Other difficulties previously discussed also appear to be diminished if we adopt this point of view. We need not regard maleness and femaleness as unit characters in heredity of the same kind as Mendelian characters of the soma. Instead of saying that the zygote composed of ovum and spermatozoon is incapable of giving rise in the male to ova, or in the female to sperms, we should hold that the gametocytes ultimately give rise to ova or to sperms according to the metabolic processes set up and maintained in them through their successive cell-divisions under the influence of the double or single X chromosome. There still remains the difficulty of explaining why the male gametocytes after reduction develop into similar sperms, with their heads and long flagella, although half of them possess one X chromosome each and the other half none. We can only suppose that the final development of the sperms is the result of the presence of the single X chromosome in the successive generations of male gametocytes before the reduction divisions.

The Mendelian theory of s.e.x-heredity a.s.sumed that in the reduction divisions the two s.e.x-characters, maleness and femaleness, were segregated in the same way as a pair of somatic allelomorphs, but the words maleness and femaleness expressed no real conceptions. The view above suggested merely attempts to bring our real knowledge of the difference between ovum and sperm into relation with our real knowledge of the s.e.x-chromosomes and their behaviour in reduction and fertilisation.

CHAPTER III

Influence Of Hormones On Development Of Somatic s.e.x-Characters

We have next to consider what are commonly called secondary s.e.xual characters. These are characters or organs more or less completely limited to one s.e.x. When we distinguish in the higher animals the generative organs or gonads on the one hand from the body or soma on the other, we see that all differences between the s.e.xes, except the gonads, are somatic, and we may call them somatic s.e.xual characters. The question at once arises whether the soma itself is s.e.xual, that is to say, whether on the a.s.sumption that the s.e.x of the zygote is already determined before it begins to develop, the somatic cells as well as the gametocytes are individually and collectively either male or female. In previous discussions of the subject I have urged that the only meaning of s.e.x was the difference between the megagamete or ovum, and the microgamete or sperm. But if the zygote, although compounded of ovum and sperm, is predestined to give rise in the gametes descended from it, either to sperms only or to ova only, it may be suggested that all the somatic cells descended from the zygote are likewise either male or female, although they do not give rise to gametes. It is evident, however, that the somatic cells, organs, and characters do not differ necessarily or universally in the two s.e.xes. On the one hand, we have extraordinary and very conspicuous peculiarities in the male, entirely absent in the female, such as the antlers of stags, and the vivid plumage of the gold pheasant; on the other we have the s.e.xes externally alike and only distinguished by their s.e.xual organs, as in mouse, rabbit, hare, and many other Rodents, most Equidae, kingfisher, crows and rooks, many parrots, many Reptiles, Amphibia, Fishes, and invertebrate animals. In the majority of fishes, in which fertilisation is external and no care is taken of the eggs or young, there are no somatic s.e.xual differences. Moreover, somatic s.e.xual characters where they do occur have no common characteristics either in structure or position in the body. It may be said that any part of the soma may in different cases present a s.e.x-limited development. In the stag the male peculiarity is an enormous development of bone on the head, in the peac.o.c.k it is the enlargement of the feathers of the tail. In some birds there are spurs on the legs, in others spurs on the wings. It is no explanation, therefore, to say that these various organs and characters are the expression of s.e.x in the somatic cells.

As I pointed out in my _s.e.xual Dimorphism_ (1900), the common characteristic of somatic s.e.xual characters is their adaptive relation to some function in the s.e.xual habits of the species in which they occur.

There is no universal characteristic of s.e.x except the difference between the gametes and the reproductive organs (gonads) in which they are produced. All other differences, therefore, including genital ducts and copulatory or intromittent organs, are somatic. When we examine these somatic differences we find that they can be cla.s.sified according to their relation to fertilisation and reproduction, including the care or protection of the offspring. The precise cla.s.sification is of no great importance, but we may distinguish the following kinds to show the chief functions to which the characters or organs are adapted.

1. GENITAL DUCTS AND INTROMITTENT ORGANS.--According to the theory of the coelom which we owe to Goodrich, in all the coelomata the coelom is primarily the generative cavity, on the walls of which the gametocytes are situated, and the coelomic ducts are the original genital ducts. In Vertebrates we find two such ducts in both s.e.xes in the embryo, originally formed apparently by the splitting of a single duct. In the male one of these ducts becomes connected with the testis while the other degenerates: the one which degenerates in the male forms the oviduct in the female, while the one which is functional in the male degenerates in the female.

Intromittent organs are formed in all sorts of different ways in different animals. In Elasmobranchs (sharks and skates) they are enlarged portions of the pelvic fins, and therefore paired. In Lizards they are pouches of the skin at the sides of the cloacal opening. In Mammals the single p.e.n.i.s is developed from the ventral wall of the cloaca. In Crustacea certain appendages are used for this function. There are a great many animals, from jelly-fishes to fishes and frogs, in which fertilisation is external, and there are no intromittent organs at all.

2. ORGANS FOR, CAPTURING OR HOLDING THE FEMALE: for example, the thumb-pads of the frog, and a modification of the foot in a water-beetle.

Certain organs on the head and pelvic fins of the Chimaeroid fishes are believed to be used for this purpose.

3. WEAPONS.--Organs which are employed in combats between males for the exclusive possession of the females. For example, antlers of stags, horns of other Ruminants, tusks of elephants, spurs of c.o.c.ks and Phasiamidae generally, horns and outgrowths in males of Reptiles and many Beetles, probably used for this purpose.

4. ALLUREMENTS.--Organs or characters used to attract or excite the female. These might be called the organs of courtship, such as the peac.o.c.k's tail, the plumes of the birds-of-paradise, and the brilliant plumage of humming birds and many others. The song of birds is another example, and sound is produced in many Fishes for a similar purpose.

5. ORGANS FOR THE BENEFIT OF THE OFFSPRING: for example, the extraordinary pouches in which the eggs are developed in certain Frogs. In the South American species, _Rhinoderma darwinii_, the enlarged vocal sacs are used for this purpose. Pouches with the same function are developed in many animals, for instance in Pipe-fishes and Marsupials. Abdominal appendages are enlarged in female Crustacea for the attachment of the eggs, the abdomen also being larger and broader.

The argument in favour of the Lamarckian explanation of the evolution of these adaptive characters is the same as in the case of adaptations common to both s.e.xes, namely that in every case the function of the organs and characters involves special irritations or stimulations by external physical agents. Mechanical irritation, especially of the interrupted kind, repeated blows or friction causes hypertrophy of the epidermis and of superficial bone. I have stated this argument and the evidence for it in some detail in my volume on _s.e.xual Dimorphism_. It is one of the most striking facts in support of this argument that the hypertrophied plumage which const.i.tutes the somatic s.e.xual character of the male in so many birds is habitually erected by muscular action for the purpose of display in the s.e.xual excitement of courtship. I doubt if there is a single instance in which the male bird takes up a position to present his ornamental plumage to the sight of the female without a special erection and movement of the feathers themselves. Such a stimulation must affect the living epidermic cells of the feather papilla. Even supposing that the feather is not growing at the time, it is probable, if not certain, that the stimulation will affect the papilla at the base of the feather follicle, so as to cause increased growth of the succeeding feather. But we have no reason to believe that erection in display occurs only when the growth of the feathers is completed, still less that it did so always at the beginning of the evolution.

The antlers of stags are the best case in favour of the Lamarckian view of the evolution of somatic s.e.xual characters. The shedding of the skin ('velvet') followed by the death of the bone, and its ultimate separation from the skull, are so closely similar to the pathological processes occurring in the injury of superficial bones, that it is impossible to believe that the resemblance is only apparent and deceptive. In an individual man or mammal, if the periosteum of a bone is destroyed or removed the bone dies, and is then either absorbed, or separated from the living bone adjoining, by absorption of the connecting part. In the stag both skin and periosteum are removed from the antler: probably they would die and shrivel of their own accord by hereditary development, but as a matter of fact the stag voluntarily removes them by rubbing the antler against tree trunks, etc. When the bone is dead the living cells at its base dissolve and absorb it, and when the base is dissolved the antler must fall off.

The adaptive relation is not the only common characteristic of these somatic s.e.xual characters. Another most important fact is not only that they are fully developed in one s.e.x, absent or rudimentary in the other, but that their development is connected with the functional maturity and activity of the gonads. There is usually an early immature period of life in which the male and female are similar, and then at the time of p.u.b.erty the somatic s.e.xual characters of either s.e.x, generally most marked in the male, develop. In some cases, where the activity of the gonads is limited to a particular season of the year, the s.e.xual characters or organs are developed at this season, and then disappear again, so that there is a periodic development corresponding to the periodic activity of the testes or ovaries. Stags have a limited breeding or 'rutting' season in autumn (in north temperate regions), and the antlers also are shed and developed annually. In this case we cannot a.s.sert that the development of the antler takes place during the active state of the testes. The antlers are fully developed and the velvet is shed at the commencement of the rutting season, and development of the antlers takes place between the beginning of the year and the month of August or September. In ducks and other birds there is a brilliant male-breeding plumage in the breeding season which disappears when breeding is over, so that the male becomes very similar to the female. In the North American fresh-water crayfishes of the genus Cambarus there are two forms of males, one of which has testes in functional activity, while in the other these organs are small and quiescent: the one form changes into the other when the testes pa.s.s from the one condition to the other.

It has long been known that the development of male s.e.x-characters is profoundly affected by the operation of castration. The removal of the testes is most easily carried out in Mammals, in consequence of the external position of the organs in these animals, and the operation has been practised on domesticated animals as well as on man himself from very ancient times. The effect is the more or less complete suppression of the male insignia, in man, for example, the beard fails to develop, the voice does not undergo the usual change to lower pitch which takes place at p.u.b.erty, and the eunuch therefore has much resemblance to the boy or woman. Many careful experimental researches have been made on the subject in recent years. The consideration of the subject involves two questions: (1) What are the exact effects of the removal of the gonads in male and female? (2) By what means are these effects brought about, what is the physiological explanation of the influence of the gonads on the soma?

I have quoted the evidence concerning the effects of castration on stags in my _s.e.xual Dimorphism_ and in my paper on the 'Heredity of Secondary s.e.xual Characters.' [Footnote: _Archiv fur Entwicklungesmechanik_, 1908.]

When castration is performed soon after birth a minute, simple spike antler is developed, only two to four inches in length: it remains covered with skin, is never shed, and develops no branches. When the operation is performed on a mature stag with antlers, the latter are shed soon after the operation, whether they have lost their velvet or not. In the following season new antlers develop, but these never lose their velvet or skin and are never shed.

CASTRATION IN FOWLS

The removal of the testes from young c.o.c.ks has been commonly practised in many countries, _e.g._ France, capons, as such birds are called, being fatter and more tender for the table than entire birds. The actual effect, however, on the secondary s.e.xual characters has not in former times been very definitely described. The usual descriptions represent the castrated birds as having rather fuller plumage than the entire birds; but the comb and wattles are much smaller than in the latter, more similar to those of a hen. It is stated that the capon will rear chickens, though he does not incubate, and that they are used in this way in France.

The most precise of the statements on the subject by the earlier naturalists is that of William Yarrell [Footnote: _Proc. Linn. Soc., 1857.] (1857), who writes as follows:--

'The capon ceases to crow, the comb and gills do not attain the size of those parts in the perfect male, the spurs appear but remain short and blunt, and the hackle feathers of the neck and saddle instead of being long and narrow are short and broadly webbed. The capon will take to a clutch of chickens, attend them in their search for food, and brood them under his wings when they are tired.'

It would naturally be expected, on the a.n.a.logy of the case of stags, that when a young c.o.c.k was completely castrated all the male secondary characters would be suppressed, namely, the greater size of the comb and wattles in comparison with the hen, the long neck hackles, and saddle hackles, long tail feathers, especially the sickle-feathers, and the spurs. As a matter of fact, the castrated specimen usually shows only the first of these effects to any conspicuous degree. The comb and wattles of the capon are similar to those of the hen, but he still has the plumage and the spurs of the entire c.o.c.k. Many investigators have made experiments in relation to this subject, and most of them have found that complete castration is difficult, and that portions of the testes left in the bird during the operation become grafted in some other position either on the parietal peritoneum, or on that covering the intestines, and produce spermatozoa, which, of course hare no outlet. In such cases the secondary male characters may fee more or less completely developed. Thus Shattock and Seligmann (1904) state that ligature of the vas deferens made no difference to the male characters, and that after castration detached fragments were often left in different positions as grafts, when the secondary characters developed. In one particular case only a minute nodule of testicular tissue showing normal spermatogenesis was found on post mortem examination attached to the intestine. In this bird there was no male development of comb or wattles, a full development of neck hackles, a certain development of saddle hackles, a few straggling badly curved feathers in the tail and short blunt spurs on the legs. Lode [Footnote: _Wiener klin. Wochenschr._, 1895.] (1895) found that testes could easily be transplanted into subcutaneous tissue and elsewhere, and that the male characters then developed normally. Hanau [Footnote: _Arch.

f. ges. Physiologie_, 1896.] (1896) obtained the same result.

The question, however, to what degree the male characters of the c.o.c.k are suppressed after complete castration is not so definitely answered in the literature of the subject. Shattock and Seligmann in their 1904 paper make no definite statement on the subject. Rieger (1900), Selheim (1901), and Foges [Footnote: _Pfugers Archiv_, 1902.] (1902) state that the true capon is characterised by shrivelling of the comb, wattles, _and spurs_; poor development of the neck and tail feathers; hoa.r.s.e voice and excessive deposit of fat. Shattock and Seligmann, on the other hand, have placed in the College of Surgeons Museum the head of a Plymouth Rock which was castrated in 1901. It was hatched in the spring of that year. In December 1901 the comb and wattles were very small, the spurs fairly well developed, and the tail had a somewhat masculine appearance. In September 1902, when the bird was killed, the comb and wattles were still poorly developed, the neck hackles fairly well so; saddle hackles rather well developed; the tail contained rather loosely-grouped long sickle feathers; the spurs stout. The description states that dissection showed no trace of either t.e.s.t.i.c.l.e, and I am informed by Mr. Shattock that there were no grafts. The description ends with the conclusion that the growth of the spurs, and to a certain extent that of the long, curved sickle feathers, is not prevented by castration. With regard to the spurs this result does not agree with that of the German investigators, but it must be remembered that the latter speak only of the reduction of the spurs, not entire absence. It is important in discussing the effects of castration in c.o.c.ks to bear in mind the actual course of development of the secondary s.e.xual characters. When the chicks are first hatched they are in the down: rudimentary combs are present, wattles can scarcely be distinguished, and there is no external difference between the s.e.xes. The ordinary plumage begins to develop immediately after hatching, the primaries of the wings being the first to appear. The feathers are completely developed in about five weeks, and still there is no difference between the s.e.xes. The first s.e.xual difference is the greater size of the combs in the males, and this is quite distinct at the age of six weeks. At nine to ten weeks in black-red fowls, in which the c.o.c.ks have black b.r.e.a.s.t.s and red backs with yellow hackles, the black feathers on the breast and red on the back are gradually developing, both s.e.xes previously having been a dull speckled brown, closely similar to the adult hens. The spurs are the last of the male characters to develop, these at the age of four months being still mere nodules, scarcely, if at all, larger than the rudiments visible in adult hens. This is the age at which castration is usually performed, as at an earlier age the birds are too small to operate on successfully. It follows, therefore, that the spurs develop after castration, and it would seem that their development does not depend upon the presence of the s.e.xual organs. It is a question, however, whether castration in the c.o.c.k is ever quite complete. In the original wild species and in the majority of domesticated breeds the spurs are confined to the male s.e.x, and are typical secondary s.e.x-characters, as much so as the antlers of stags or the beard of man, yet the above discussion shows that there is some doubt whether their development is prevented as much as in other cases by the absence of the s.e.xual organs. Even if it should be proved that in supposed cases of complete castration, such as that of Shattock and Seligmann, some testicular tissue remained at the site of the testes, it would still be true that the development of the comb and wattles is more affected by the removal of the s.e.xual organs than that of the spurs or tail feathers.

My own experiments in castrating c.o.c.ks were as follows: On August 20, 1910, I operated on a White Leghorn c.o.c.k about five months old. One testis was removed, with a small part of the end broken off, but the other, after it was detached, was lost among the intestines. On the same day I operated on another about thirteen weeks old, a speckled mongrel. In this case both testes were extracted but one was slightly broken at one end, although I was not sure that any of it was left in the body. An entire White Leghorn of the same age as the first was kept as a control. On August 27 the two castrated birds had recovered and were active. Their combs had diminished in size and lost colour considerably, that of the White Leghorn was scarcely more than half as large as that of the control. Such a rapid diminution can scarcely he due to absorption of tissue, but shows that the size of the normal c.o.c.k's comb is largely due to distension with blood, which ceases when the s.e.xual organs are removed. In the following January, the second c.o.c.k, supposed to be completely castrated, was seen to make a s.e.xual gesture like a c.o.c.k, though not a complete action like an entire animal: this showed that the s.e.xual instinct was not completely suppressed. In February this same bird was seen to attempt to tread a hen, while the white one, supposed to be less perfectly emasculated, had never shown such male instinct.