THE CONTEMPORARY ~ SCIENCE SERIES- THE EVOLUTION OF SEX ‘ ste, Lids Sih la ialie baal j OP in THE CONTEMPORARY SCIENCE SER/ES. EDITED BY HAVELOCK ELLIS. BUREAU OF AMERICAN ETHNOLOGY. ee LIBRARY Mie EVOLUTION OF SEX |: J aa 4 F/ gq (Gaz? EVOLUTION OF SEX soijude BY PROFESSOR PATRICK GEDDES i AND J. ARIA WeOMS OM. With 104 I!lustrations. BUREAU OF AMERICA nx ETHNOLOGY 1897 LIBRABHY LONDON: WANK SCOmT:, LTD.- PATERNOSTER SQUARE. CERNE So SEREBINE RSs SONS: 153-157 FIFTH AVENUE, NEW YORK. 1897. PREBAGE. N course of the preparation of critical summaries, such as the articles “‘ Reproduction” or ‘‘ Sex,” contributed by one of us to the “ Encyclopzedia Britannica,” or the account of recent progress annually prepared for the Zoological Record by the other, we have not only naturally accumulated considerable material towards a general theory of the subject, but have come to take up an altered and unconventional view upon the general questions of biology, particularly upon that of the factors of organic evolution. ence this little book has the difficult task of inviting the criticism of the biological student, although primarily addressing itself to the general reader or beginner. The specialist therefore must not expect exhaustiveness, despite a good deal of small type and bibliography, over which other readers (for whose sakes technicalities have also been kept down as much as possible) may lightly skim. Our central thesis has been, in the first place, to present an outline of the main processes for the continuance of organic life with such unity as our present knowledge renders possible ; and in the second, to point the way towards the interpretation of these processes in those ultimate biological terms which physiologists are already reaching as regards the functions of individual life,—those of the constructive and destructive changes (anabolism and katabolism) of living matter or proto- plasm. sal PREFACE. But while Books I. and II. are thus the more important, and such chapters as “‘ Hermaphroditism,” ‘‘ Parthenogenesis,” “ Alternation of Generations,” have only a subordinate and comparatively technical interest, it will be seen that our theme raises nearly all the burning questions of biology. Hence, for instance, a running discussion and criticism of the speculative views of Professor Weismann, to which their very recent intro- duction to English readers* has awakened so wide an interest. At once of less technical difficulty, and in some respects even wider issues, is the discussion of Mr Darwin’s theory of sexual selection, reopened by the other leading contribution to the year’s biological literature which we owe to Mr Alfred Russel Wallace.t Besides entering this controversy at the outset of the volume, we have in the sequel attempted to show that the view taken of the processes concerned with the maintenance of the species leads necessarily to a profound alteration of our views regarding its origin, although the vast problems thus raised necessarily remain open for fuller separate treatment. It is right, however, to say that the restatement of the theory of organic evolution, for which we here seek to prepare (that not of indefi- nite but definite variation, with progress and survival essen- tially through the subordination of individual struggle and development to species-maintaining ends), leads us frankly to face the responsibility of thus popularising a field of natural knowledge from which there are so many superficial reasons to shrink, and which knowledge and ignorance so commonly conspire to veil. For if not only the utmost degeneracy be manifestly connected with the continuance of organic species, but also the highest progress and blossoming of life in all its forms, of man or beast or flower, it becomes the first practical * << FHleredity.” Oxford, 1889. + ** Darwinism.” Lond. 1889. PREFACE. Vil application of biological science not only to investigate and map out these two paths of organic progress, but to illuminate them. Hence we have attempted to indicate the application of the general organic survey, which has been our main theme, to such questions as those of human population and progress, although here, more even than elsewhere, our treatment can be at best only suggestive, not exhaustive. While limits of space have made it impossible to give the botanical side of our sub- ject its proportionate share of attention, our illustrations of the essential facts are sufficient to show the parallelism of the reproductive processes throughout nature. It remains to express our thanks to Professor F. Jeffrey Bell for some valuable suggestions while the work was passing through the press; to Mr G. F. Scott-Elliot for assistance in summarising certain portions of the literature; and to our engravers, Messrs Harry S. Percy, F. V. M‘Combie, and G. A. Morison, especially to the first-named, who has executed the great majority of our illustrations with much care and skill. PATRICK GEDDES. J. ARTHUR THOMSON. ERRATUM. On page 146, instead of present title of figure, read :— “The Process of Fertilisation.—After Boveri.” Gi@ NobaEUN: Wes: BOOK I.—MALE AND FEMALE. Pigmies and exceptions. 4. Secondary differences in colour, skin, &c. Males katabolic, females anabolic. § 5. Sexual selection : Its limits as an explanation. Postulate of extreme esthetic sensi- tiveness. Darwin and Wallace combined and supplemented. Sexual selection a minor accelerant, natura} selection a retarding action, on constitutional differentiation. CHAPTER I. PAGE THE SEXES AND SEXUAL SELECTION” - - - - 3-15 § 1. Primary and secondary sexual characters. § 2. Illustrations from Darwin. § 3. Darwin’s explanation by sexual selection. § 4. Criticisms of sexual selection— (a.) Wallace. (6.) Brooks. (c.) St George Mivart. (d.) Others. CEUAP AER Ie THE SEXES, AND CRITICISM OF SEXUAL SELECTION - of parasitism? The life-history answers. this objection. The two sexes are at first alike,—agile, and resembling most thread- worms; they become parasitic, and lose both activity and nematode form ; but the interesting fact is further, that the male recovers himself, while the female remains a victim. In other insect and worm types the same story, in less accented characters, may be distinctly read. In many crusta- ceans, again, the femalesonly are parasitic; and while this is in part explained by their habit of seeking shelter for egg-laying pur- poses, it also expresses the constitutional bias of the sex. The insect order of bee parasites (Stvepstptera) is remarkable for the completely passive and even larval character of the blind parasitic females, while the adult males are free, winged, and short-lived. Throughout the class of insects there are Female Chondracanthus, a numerous illustrations of the excellence Leta we ee of the males over the females, alike in attached just above the origin of the long egg- muscular power and sensory acuteness. SSD) GE ine Sa. The diverse series of efforts by which the = —From Claus. males of so many different animals, from cicadas to birds, sustain the love-chorus, affords another set of illustrations of pre-eminent masculine activity. Without multiplying instances, a review of the animal B 18 THE EVOLUTION OF SEX. kingdom, or a perusal of Darwin’s pages, will amply confirm the conclusion that on an average the females incline to passivity, the males to activity. In higher animals, it is true that the contrast shows rather in many little ways than in any one striking difference of habit, but even in the human species the contrast is recognised. Every one will admit that strenuous spasmodic bursts of activity characterise men, especially in youth, and among the less civilised races ; while patient con- tinuance, with less violent expenditure of energy, is as generally associated with the work of women. Both sexes of a Flea—the Jigger or Chigoe(Sarcopsylla penetrans); the female much swollen with eggs.—From Leuckart. For completeness of argument, two other facts, which will afterwards claim full discussion, may here be simply mentioned. (az) At the very threshold of sex-difference, we find that a little active cell or spore, unable to develop of itself, unites in fatigue with a larger more quiescent individual. Here, at the very first, is the contrast between male and female. (4) The same antithesis is seen, when we contrast, as we shall afterwards THE SEXES, AND CRITICISM OF SEXUAL SELECTION. 19 do in detail, the actively motile, minute, male element of most animals and many plants, with the larger passively quiescent female-cell or ovum. It is possible that the reader may urge as a difficulty against the above contrast the exceedingly familiar case of the male bees or ‘‘drones.” It must be frankly allowed that exceptions do indeed occur, though usually in conditions which afford a key to the abnormality. Thus it will be allowed that the “drones” are in a peculiar position as male members of a very complex society, in which what is practically a third sex is represented by the great body of “workers.” They are no more fair examples of the natural average of males, than the hard-driven wives of the lazy Kaffir are of the normal functions of women. Nor is the exception even here a real one, for the drone, although passive as compared with the unsexed workers, is active when compared with the extraordinarily passive queen. To the above contrast of general habit, two other items may be added, on which accurate observation is still unfortun- ately very restricted. In some cases the body temperature, which is an index to the pitch of the life, is distinctly lower in the females, as has been noted in cases so widely separate as the human species, insects, and plants. In many cases, further- more, the longevity of the females is much greater. Such a fact as that women pay lower insurance premiums than do men, is often popularly accounted for by their greater immunity from accident ; but the greater normal longevity on which the actuary calculates, has, as we begin to see, a far deeper and constitutional explanation. § 3. Szze—Among the higher animals, there are curious alternations in the preponderance of one sex over another in size. Thus among mammals and birds the males are in most cases the larger; the same is true of lizards; but in snakes the females preponderate. In fishes, the males are on an average smaller, sometimes very markedly so, even to the extent of not being half as large as their mates. Below the line, among backboneless animals, there is much greater constancy of predominance in favour of the females. Thus among insects, the more active males are generally smaller, and often very markedly ; of spiders the same is true, and the males being often very diminutive are forced to task their agility to the utmost in making advances to their unamiable mates. So again, crustacean males are often smaller than the females; and 20 THE EVOLUTION OF SEX. in many parasitic species, what have been well called “ pigmy ” males illustrate the contrast in an almost ludicrous degree. Two cases from aberrant worm types exhibit very vividly this same antithesis of size. Among the common rotifers, the males are almost always very different from the females, and much smaller. Sometimes they seem to have dwindled out of existence altogether, for only the females are known. In other cases, though present, they entirely fail to accomplish their proper function of fertilisation, and, as parthenogenesis obtains, are not only minute, but useless. In a curious green marine Relative sizes of a male and female Rotifer (Hydatina senta). —From Leunis. worm, 4ozellia, the male remains like a remote ancestor of the female. It lives parasitically on or within the latter, and is microscopic in size, measuring in fact only about one hundredth part of the length of its host and mate. Somewhat similar to the case of bonellia is that of a viviparous coccus insect (Lecanium hesperidum), where the males are very degenerate, small, blind, and wingless. In spite of this condition, we should indeed think because of it, they are very male, for even the larvee, while still within the mother, have been shown to contain fully- developed spermatozoa. It would be unfair to argue from such an extreme case as that of Bonellia alone, but there is no doubt that up to the THE SEXES, AND CRITICISM OF SEXUAL SELECTION, P43) level of amphibians at least the females are generally the larger. -This then must be taken in connection with the conclusion of the previous paragraph. A sluggish conservative habit of body tends to an increase of size; lavish expenditure of energy keeps down the accumulation of storage. Corroborative evidence will be afterwards forthcoming, as we contrast (a) the large and small spores which mark the beginnings of sex differences, or (0) the relatively large female cell or egg with the microscopic male cell or spermatozoon. Figure of the female Bonellia (from Atlas of Naples Aquarium), with its parasitic pigmy male enlarged. Apparent exceptions occur, it is true, among the higher animals. In birds and mammals the males are usually rather larger than the females. ‘This difference consists especially in larger bones and muscles. ‘The apparent exception is in part the natural result of the increased stress of external activities which are thrown upon the shoulders of the males when their mates are incapacitated by incubation and pregnancy. Further- more, we must recognise the strengthening influence of the 22 THE EVOLUTION OF SEX combats between males, and the effect produced on the accumulative constitution of the females by the increased maternal sacrifice characteristic of the highest animals. § 4. Other Characters —While it is easy to point to the general physiological import of large size and the reverse, physiology is not yet far enough advanced to afford firm foot- hold in dealing with the details of secondary sexual characters. It is only possible to point out the path which will eventually lead us to their complete rationale. This path will appear less vague if reverted to after some of the succeeding chapters have been grasped. The point of view is simple enough. The agility of males is not a special adaptation to enable that sex to exercise its functions with relation to the other, but is a natural characteristic of the constitutional activity of maleness ; and the small size of many male fishes is not an advantage at all, but simply again the result of the contrast between the more vegetative growth of the female and the costly activity of the male. So, brilliancy of colour, exuberance of hair and feathers, activity of scent-glands, and even the development of weapons, are not, and cannot be (except teleologically), explained by sexual selection, but in origin and continued development are outcrops of a male as opposed to a female constitution. ‘To sum up the position in a paradox, all secondary sexual characters are at bottom primary, and are expressions of the same general habit of body (or to use the medical term, diathesis), as that which results in the production of male elements in the one case, or female elements in the other.* Three well-known facts must be recalled to the reader’s mind at this point; and firstly, that in a great number of cases the secondary sexual characters make their appearance step by step with sexual maturity itself. When the animal—be it a bird or insect—becomes emphatically masculine, then it is that these minor outcrops are exhibited. Thus the male bird of paradise, eventually so resplendent, is usually in its youth comparatively dull and female-like in its colouring and plumage. Very often too, whether in the wedding-robe of male fishes or in the scent-glands of mammals, the character rises and wanes in the same rhythm as that of the reproductive periods. It is impossible not to regard at least many of the * That Mr Wallace has adopted the same explanation of the different sexual characters in his new book, has been already pointed out (see p. II, note). THE SEXES, AND CRITICISM OF SEXUAL SELECTION. 23 secondary sexual characters as part and parcel of the sexual diathesis, —as expressions for the most part of exuberant maleness. Secondly, when the reproductive organs are removed by castration, the secondary sexual characters tend to remain undeveloped. Thus, as Darwin notes, stags never renew their antlers after castration, though normally of course they renew them each breeding season. ‘The reindeer, where the horns occur on the females as well, is an interesting excep- tion to the rule, for after castration the male still renews the growth. This however merely indicates that the originally sexual characters have become organised into the general life of the body. In sheep, antelopes, oxen, &c., castration modifies or reduces the horns ; and the same is true of odoriferous glands. The parasitic crustacean Sacculina has been shown by Delage to effect a partial castration of the crabs to which it fixes itself, and the same has been observed by Giard in other cases. In two such cases an approximation to the female form of appendage has been observed. Lastly, in aged females, which have ceased to be functional in reproduction, the minor peculiarities of their sex often disappear, and they become liker males, both in structure and habits,—witness the familiar case of “crowing hens.” From the presupposition, then, of the intimate connection between the sexuality and the secondary characters (which is indeed everywhere allowed), it is possible to advance a step further. Thus in regard to colour, that the male is usually brighter than the female is an acknowledged fact. But pig- ments of many kinds are physiologically regarded as of the nature of waste products. Such for instance is the guanin, so abundant on the skin of fishes and some other animals. Abundance of such pigments, and richness of variety in related series, point to pre-eminent activity of chemical processes in the animals which possess them. ‘Technically expressed, abundant pigments are expressions of intense metabolism. But pre- dominant activity has been already seen to be characteristic of the male sex; these bright colours, then, are often natural to maleness. In a literal sense animals put on beauty for ashes, and the males more so because they are males, and not primarily for any other reason whatever. We are well aware that, in spite of the researches of Krukenberg, Sorby, MacMunn, and others, our knowledge of the physiology of many of the pigments is still very scanty. Yet in many cases, alike among 24 THE EVOLUTION OF SEX. plants and animals, pigments are expressions of disruptive processes, and are of the nature of waste products; and this general fact is at present sufficient for our contention, that bright colouring or rich pigmenting is commonly a natural expression of the male constitution. For the red pigment so abundant in the female cochineal insect, which appears to be of the nature of a reserve and not a waste product, and for similar occurrences, due exception must be made. In the same way, the skin eruptions of male fishes at the spawning season seem more pathological than decorative, and may be directly connected with the sexual excitement. One instance of the way in which the reproductive maturity is known to effect a by no means obviously related result may be given. Every field naturalist knows that the male stickleback builds a nest among the weeds, and that he weaves the material together by mucous threads secreted from the kidneys. ‘The little animal is also known to have strong passions; it is polygamous in relation to its mates, and most pugnacious in relation to its rivals. Professor Mobius has shown that the male reproductive organs (or testes) become very large at the breeding season, and that they press in an abnormal way upon the kidneys. This encroachment produces a pathological condition in the kidneys, and the result is the formation of a mucous secretion, somewhat similar to what occurs in renal disease in higher forms. To free itself from the irritant pressure of this secretion, the male rubs itself against external objects, most conveniently upon its nest. Thus the curious weaying instinct does not demand or find rationale in the cumulative action of natural selection upon an inexplicable variation, and is traced back to a pathological and mechanical origin in the emphatic maleness of the organism. The line of variation being thus given, it is of course conceiv- able that natural selection may have accelerated it. So too, though again the physiological details are scanty, the superabundant growth of hair and feathers may be interpreted, in some measure through getting rid of waste products, for we shall see later how local katabolism favours cell multiplication. Combs, wattles, and skin excrescences point to a predominance of circulation in the skin of the feverish males, whose tempera- tures are known in some cases to be decidedly higher than those of the females. Even skeletal weapons like antlers may be similarly interpreted ; while the exaggerated activity of the scent-glands is another expedient for excreting waste. THE SEXES, AND CRITICISM OF SEXUAL SELECTION, 25 In regard to horns, feathers, and the like, in association with vigorous circulation, two sentences from Rolph may be quoted :—‘‘ The exceedingly abundant circulation, which peri- odically occurs in the at first soft frontal protuberances of stags, admits and conditions the colossal development of horn and delicate ensheathing velvet. . . . In the same way, the rich flow of blood in the feather papillee conditions the immense growth of the feathers, . . . and the same is true of hairs, spines, and teeth.” Male (c), Worker (4), and Queen (a) Ant.—From Chamibers’s Eneyc., after Lubbock. Some of the even subtler differences between the sexes are of interest in illustrating the general antithesis. Thus in the love-lights of the Italian glow insect (Luciola), the colour is said to be identical in the two sexes, and the intensity is much the same. That of the female, however, who is in other repects rather male-like in her amatory emotions, is more restricted. It is interesting further to notice, that the rhythm of the light in the male is more rapid and the flashes are briefer, while that of the female is longer and the flashes more distant and tremu- lous. This illustration may thus serve, in conclusion, as a literally illumined index of the contrasted physiology of the SEXES. $5. Sexual Selection: tis Limit as an Explanation.—We are now in a better position to criticise Mr Darwin’s theory. On his view, males are stronger, handsomer, or more emo- tional, because ancestral forms happened to become so in a slight degree. In other words, the reward of breeding success 26 THE EVOLUTION OF SEX.. gradually perpetuated and perfected a casual advantage. According to the present view, males are stronger, handsomer, or more emotional, simply because they are males,—z.e., of more active physiological habit than their mates. In phrase- ology which will presently become more intelligible and concrete, the males live at a loss, are more kafabolic,—dis- ruptive changes tending to preponderate in the sum of changes in their living matter or protoplasm. The females, on the other hand, live at a profit, are more azabolic,—constructive processes predominating in their life, whence indeed the capacity of bearing offspring. No one can dispute that the nutritive, vegetative, or self- regarding processes within the plant or animal are opposed to the reproductive, multiplying, or species-regarding processes, as income to expenditure, or as building up to breaking down. But within the ordinary nutritive or vegetative functions of the body, there is necessarily a continuous antithesis between two sets of processes,—constructive and destructive metabolism. The contrast between these two processes is seen throughout nature, whether in the alternating phases of cell life, or of activity and repose, or in the great antithesis between growth and reproduction; and it is this same contrast which we recognise as the fundamental difference between male and female. The proof of this will run through the work, but our fundamental thesis may at once be roughly enunciated in a diagrammatic expression (which in its present form we owe to our friend Mr W. E. Fothergill) :— SUM OF FUNCTIONS. Nutrition. Reproduction. / we Wi, if Ji Uf / Anabolism. Katabolism. Female. Male. Here the sum-total of the functions are divided into nutritive and reproductive, the former into anabolic and THE SEXES, AND CRITICISM OF SEXUAL SELECTION. 27 katabolic processes, the latter into male and female activities,—so far with all physiologists, without exception or dispute.* Our special theory lies, however, in suggesting the parallelism of the two sets of processes,—the male reproduction is associated with preponderating katabolism, and the female with relative anabol- ism. In terms of this thesis, therefore, both primary and second- ary sexual characters express the fundamental physiological bias characteristic of either sex. Sexual selection resembles artificial selection, but the female takes the place of the human breeder; it resembles natural selection, but the selective females and the com- bative males represent a role filled in the larger case by the foster- ing or eliminating action of the environment. As a special case of natural selection, Darwin’s minor theory is open to the objection of being teleological, z.¢., of accounting for structures in terms of a final advantage. It is quite open to the logical critic to urge, as a few have done, that the structures to be explained have to be accounted for before, as well as after, the stage when they were developed enough to be useful. The origin, or in other words the fundamental physiological import, of the structures, ‘must be explained before we have a complete or adequate theory _ of organic evolution. Apart from this logical insufficiency, the theory of sexual selection is open to many minor objections, with some of which Darwin himself dealt, as is mentioned in the preceding historical chapter. One detailed objection which seems serious may also be urged. The evolution of coloured markings by selective pre- ference carries with it the postulate of a certain level of zesthetic taste and critical power in the female, and this not only very high and very scrupulous as to details, but remaining permanent as a standard of fashion from generation to generation,—large assumptions all, and scarcely verifiable in human experience. Yet we cannot suppose that Mr Darwin considered the human female as peculiarly undeveloped. It is true, doubtless, that * The reader whose physiological studies may not have been so recent as to familiarise him with that conception of all physiological processes as finding their ultimate expression in the metabolism (anabolism and katabolism) of protoplasm, will easily place himself in a position to check our argument (often indeed, we trust to carry our interpretation of sex into still further detail) by starting from the exposition of this doctrine in Dr Michael Foster’s article, ‘‘ PHysioLoGcy,” in the Zxcyclopedia Britannica, or with Dr Burdon Sanderson’s Presidential Address to Section D, British Association, 1889. The essential conception will, however, become clearer as we proceed (see pp. 89, 124). 28 THE EVOLUTION OF SEX. both insects and birds have so far and increasingly become educated in such sensitiveness; but when we consider the com- plexity of the markings of the male bird or insect, and the slow gradations from one stage of perfection to another, it seems difficult to credit birds or butterflies with a degree of eesthetic development exhibited by no human being without both special gesthetic acuteness and special training. Moreover, the butter- fly, which is supposed to possess this extraordinary development of psychological subtlety, will fly naively to a piece of white paper on the ground, and is attracted by the primary esthetic stimulus of an old-fashioned wall-paper, not to speak of the gaudy and monotonous brightness of some of our garden flowers. Thus we have the further difficulty, that we must suppose the female butterfly to have a double standard of taste, one for the flowers which she and her mate both visit, the other for the far more complex colouring and markings of the maies. And even among birds, if we take those unmistakable hints of real awakening of the eesthetic sense which are exhibited by the Australian bowerbird or by the common jackdaw in its fondness for bright objects, how very rude is this taste compared with the critical examination of infinitesimal variations of plumage on which Darwin relies. Is not, therefore, his essential supposition too glaringly anthropomorphic? Again, the most beautiful males are often extremely com- bative; and on the conventional view this is a mere coin- cidence, yet a most unfortunate one for Mr Darwin’s view. Battle thus constantly decides the question of pairing, and in cases where, by hypothesis, the female should have most choice, she has simply to yield to the victor. On our view, however, combative energy and sexual beauty rise favz passu with male katabolism. Or again, in the Zxeas group of the genus Fapzlzo, Darwin notes how there are frequent gradations in the amount of dif- ference between the sexes. Sometimes the sexes are alike dull, where we should have to suppose the eesthetic perception must somehow have been lost or inhibited; sometimes the females are dull and the males splendid,—for Darwin, an example of the result of sexual esthetic perception, this of an exquisitely subtle kind however, and without proportionate cerebral en- largement. In a third set of cases, both sexes are splendid, which would suggest logically that the male in turn had acquired a taste for splendour. But such cases, which usually need more THE SEXES, AND CRITICISM OF SEXUAL SELECTION. 29 or less cumbrous additional hypothesis of inheritance and so on to explain them, are intelligible enough if we regard them as illustrations of increasing katabolism throughout a series of species. The third set may be supposed to be more male or katabolic than the first, while the second set are midway; although it may be freely granted a knowledge of the habits, size, &c., of the particular species, would be necessary to verify the legitimacy of this interpretation in this particular case.* It is necessary once more to turn to the contrast between the positions of Darwin and Wallace. According to Darwin, sexual selection, for love’s sake, has accelerated the males into gay colouring; according to Wallace, natural selection, for safety’s sake, has retarded the females (birds or butterflies) and kept them inconspicuously plain. It is no longer difficult to establish a compromise. ‘The true view seems to be, that both sexes have differentiated towards their respective goals, but the males faster, because so katabolic; the limits are constantly being fixed by natural selection in Wallace’s cases, and as constantly increased by sexual selection in Darwin’s. ‘There is, in fact, no reason why both should not be admitted as minor factors ; but the greater part of the explanation is to be found in the view above stated, viz., in the physiological constitution of males and females themselves. In short, the present position allows some truth in both these conclusions, but regards gay colouring as the expression of the predominantly katabolic or male sex, and quiet plainness as equally natural to the predominantly anabolic females. On this view, too, we are able to restate part of the position emphasised by Brooks. ‘The greater variability of the males is indeed natural, if they be the more katabolic sex. In preponderant katabolism, the combinations and permutations of molecules which constitute variation, are necessarily more probable than in the quiescent, passive, or anabolic females. No special theory of heredity is required,—the males transmit the majority of variations, because they have most to transmit. At a later stage something more will be said of natural selection, and its limits as an explanation of facts. But it * For a discussion of the progressive development of colouring and markings, whether in butterflies or mammals, the reader may be referred to the works of Professor Eimer, and especially to his forthcoming work on Lepidoptera. Reference should also be made to Weismann’s ‘‘ Studies in the Theory of Descent,” for a discussion of the markings of caterpillars and butterflies. 30 THE EVOLUTION OF SEX. is here desirable to emphasise, that just as we admit the importance of sexual selection as a minor accelerant in the differentiation of the sexes, so we are bound to recognise that natural selection is also continually in operation as a check to a divergence of the sexes which would otherwise tend to become extreme. If this retarding influence of natural selec- tion on the evolutionary process were not continually present, we should find cases like bonellia and the rotifers much commoner than they are among animals. But it is an error to exaggerate this limiting action into an explanation of the process itself. It should also be noted, that both the retarding action of natural selection, and the accelerant action of sexual selection, become of increasing importance as we ascend the series. And thus, indeed, we are impelled towards a heresy which, as we shall see later, has bearings against the theory of natural selection, which overpass the limits of our present theme. Postscript.—Dr T, W. Fulton, Naturalist to the Scottish Fishery Board, has been good enough to furnish us with some of his results on the size and numerical proportions of male and female fishes. (1.) The females are usually considerably more numerous than the males, and never less numerous except in the angler and the cat-fish. The proportions of females to males among flat-fishes ranges from about I : I in the flounder, to about 12:1 in the long rough dab. Among ‘‘round” fishes the same proportion varies from about 3: 2inthe cod, tog: 2inthe common gurnard. (2.) The female is longer and larger amongall the flat-fishes, sometimes by as much as 30 per cent. In cod, haddock, angler, and cat-fish, the males are larger, while in the whiting the females are slightly larger, and in the common gurnard decidedly so. The subject is being worked up by the above- named naturalist, and cannot fail to yield very valuable results. THE SEXES, AND CRITICISM OF SEXUAL SELECTION. 31 SUMMARY. 1-3. A broader basis must be sought from which to understand the differences between the sexes. A general survey shows, that the males are more active in habit, the females more passive ; that the males tend to be smaller and to have a higher body-temperature, while the females tend to be larger and to live longer. 4. The close association of secondary sexual characters with the reproductive function, is shown in the period or in the periodicity of their development, in the effects of castration, in the peculiarities of aged females, &c. Richer pigmentation, and other male characteristics, are to be interpreted as expressions of the katabolic predominance in the con- stitution of males, as opposed to the anabolic preponderance of the females. 5 5. Sexual selection, as an explanation of secondary sexual characters, is limited, by being teleological rather than etiological, does not account for origins nor incipient stages, postulates subtle zesthetic sensitiveness, and is beset by numerous minor difficulties. Yet the opposed positions of Darwin and Wallace both emphasise indubitable facts; while the criticisms of Mivart, the theory of Brooks, and the suggestions of Rolph, Mantegazza, and others, lead on towards a deeper analysis. The general conclusion reached, recognises sexual selection (so far with Darwin) as a minor accelerant, natural selection (so far with Wallace) as a retarding ‘‘ brake,” on the differentiation of sexual characters, which essentially find a con- stitutional or organismal origin in the katabolic or anabolic diathesis which preponderates in males and females respectively. LITERATURE. BROOKS, DARWIN, MIVART, WALLACE.—As before. Ermer, G. H. T.—Die Enstehung der Arten auf Grund von Vererben erworbener Eigenschaften, nach den Gesetzen organischen Wachsens. Jena, 1888. GEDDEs, P.—Articles Reproduction, Sex, Variation and Selection. Encycl. Brit. Also on the Theory of Growth, Reproduction, Sex, and Heredity. Proc. Roy. Soc. Edin. 1885-6. Ropu, W. H.—Biologische Probleme. Leipzig, 1884. WEISMANN, A.—Studies in the Theory of Descent (Meldola’s Transla- tion). London, 1880-82. WaLttace, A. R.—Darwinism. London, 1889. CAE AW EE Reena THE DETERMINATION OF SEX (Ayfotheses and Observations). So far the differences between the sexes as observed in adult forms. Attention must now be turned to the origin of sex itself in the individual organism. ‘The historic beginning of sex will be discussed at a later stage; the present problem concerns the factors which determine whether any given organism will develop into a male or into a female. The question, in other words, is that usually known as the determination of sex. $1. The Period at which the Sex ts Determined.—Every organism, whether male or female, develops from a fertilised egg-cell, apart of course from the occurrence of asexual and parthenogenetic reproduction. This material, which in one case develops into a male, in another into a female, is, so far as our experience can go, always the same ; and w/em the sex of the organism is absolutely decided, is a question to which no general answer can be given. In the higher animals (birds and mammals) it is possible at quite an early date in embryonic life to tell whether the young organism will turn into a male or a female, though in the very earliest stages it is impossible to determine whether the rudiment of the reproductive organs is going to become a testis or anovary. But in lower vertebrates, such as frogs, the period of embryonic indifference is greatly prolonged ; and it seems certain that a hatched tadpole, even after a tendency towards, say maleness, has actually arisen, may in certain conditions have this altered in the opposite direction. Among invertebrates, the sexual organs are often late in acquiring definite predominance in favour of either sex,— that is, the period of undecided indifference is, as one would expect, usually much longer. The factors which are influential in determining sex are numerous, and come into play at different periods, so that it is quite possible for a germ-cell to have its future fate more than once changed. The constitution of the mother, the nutrition of the ova, the constitution of the father, the state of THE DETERMINATION OF SEX, 33 the male element when fertilisation occurs, the embryonic nutrition, and even the larval environment in some cases, these and yet other factors have all to be considered. Some observations by Laulanié as to the embryonic organs are of interest in this connection. He distinguishes both in birds and mammals three stages in the individual development of the reproductive organs. These he calls (1) Germiparity, (2) Hermaphroditism, (3) Differentiated Unisexuality ; and regards them as parallel to the stages of historic evolu- tion. Even for the first stage, however, when the elements are still very primitive, he would not allow the accuracy of the terms neutrality or indifference. The elements in both sexes are almost similar, but yet their future fate has been decided. Sutton has also emphasised his conviction, that in the individual development a state of embryonic hermaphroditism obtains, and main- tains that one set of elements predominates over the other in the establish- ment of the normal unisexual state. Ploss and others take up a similar position in regard to an early hermaphrodite state. It can only be concluded, that the higher the organism is in the series the earlier is its sexual fate sealed ; and that it is only in lower vertebrates, and among backboneless animals, that we can speak of prolonged neutrality of sex, or embryonic hermaphro- ditism. § 2. Answers to the Question: What Determines Sex ?2—To the question what settles whether an organism shall develop into a male or into a female, many and varied answers have been given. At the beginning of the last century, the theories of sex were estimated at so many as five hundred, and they have gone on increasing. It is evident that even an enumeration of these is not possible, nor is it indeed desirable. As in so many other cases, our ideas respecting the determination of sex have been looked at in three different ways. For the theologian, it was enough to say that “‘God made male and female.” In the period of academic metaphysics, still so far from ended, it was natural to refer to “‘inherent properties of maleness and femaleness ;” and it is still a popular “explanation” to invoke undefined “natural tendencies” to account for the production of males or females. ‘This mode of treatment, it need not be said, is being abandoned by biologists. It is recognised that the problem is one for scientific analysis; thus the constitu- tion, age, nutrition, and environment of the parents must be especially considered. These investigations, which are mainly restricted to observation and statistics, will be first noticed ; the more experimental researches, and the general conclusions, will be discussed in the next chapter. That the final physiological ex- planation is, and must be, in terms of protoplasmic metabolism, we must again, however, remind the reader (see p. 27, note). C 34 THE EVOLUTION OF SEX. § 3. The theory that there are two kinds of ova, respectively destined to develop into males or females, is more than a mere begging of the question. ‘The constitution of the ovum is undoubtedly a fact of primal importance, but we must also recognise that what is virtually decided at this early stage may be counteracted by later influences of an opposite character. The hypothesis of two kinds of ova was advanced, for example, by B. S. Schultze, but as the grounds for his views are not admitted as correct, only its existence need be noticed till more observations are forthcoming. § 4. Numerous authors have attached great importance to the process of fertilisation as a determinant of the sex. One of the most crude positoins has been that of Canestrini, who ascribed the determination of sex to the number of sperms entering the ovum :—The more sperms, the greater the tendency to male offspring. It has, however, been shown by Fol, Pfliiger, Hertwig, and others, that “‘nolyspermy,” or the entrance of more than one sperm, is extremely rare, is in fact generally impossible, and when it does in rare conditions occur, indicates a pathological condition of the egg-cell, and tends to produce abnormalities. Pfliiger diluted the seminal fluid of male frogs, and found that no change resulted in the normal numerical proportion of the sexes. The case of drones, furthermore, where male are known to arise from unfertilised ova, is a familiar example, exactly counter to Canestrini’s proposition. which may in fact be dismissed as wholly untenable. § 5. Zime of Fertilisation.—With greater weight various authorities have insisted upon the time of fertilisation. Thus, according to Thury (1863), followed by Diising (1883), an ovum fertilised soon after liberation tends to produce a female, while an older ovum will rather develop into a male. Asa practical breeder Thury claimed to determine the sex of cattle upon this principle ; Cornaz and Knight have both practically confirmed this ; while Girou has pointed out, that female flowers fertilised as soon as they were able to receive pollen tended to produce female offspring. Hertwig has also shown that, the internal phenomena of fertilisation vary somewhat with the age of the ovum at the time. MHensen is inclined to accept the general accuracy of Thury’s conclusion, but extends it to the male element as well. ‘‘ A very favourable condition in both ovum and sperm will probably lead to the formation of a female.” ‘* According to its condition, a sperm may either insufficiently corroborate the favourable state of the ovum, or constitutionally strengthen an ovum less satisfactorily conditioned.” § 6. Age of Parents.—Hofacker (1823) and Sadler (1830) independently published a body of statistics, each including about 2,000 births, in favour of the generalisation that when the male parent is the older the offspring are preponderatingly male; while if the parents be of the same age, or a fortiorvt if the male parent be the younger, female offspring appear in increasing majority. This conclusion, generally known as Hofacker’s and Sadler’s law, has received both confirmation and perplexing contradiction. It has been confirmed by Gohlert, Boulenger, Legoyt, and others, and by THE DETERMINATION OF SEX. 35 some breeders of stock and birds, but is denied by other practical authorities, and directly contradicted by the recent statistics of Stieda, from Alsace-Lorraine, and of Berner, from Scandinavia. Summary of Statistics bearing on Relative Number of Males and Females. Father Father of Father pees Now of older. equal age. younger. ‘ween Git Observer. Births Locality. | Proportion | Proportion | Proportion Males to Remarks. : of Males to | of Males to | of Males to ae 100 Females. |100 Females. |100 Females.| females. Hofacker} 1,996] Tiibingen| 117.8 92.0 go-6 107-5 Sadler 2,068 | England T21.4 94.8 86.5 114.7 Gohlert 4,584 108. 93-3 82.6 105.3 Legoyt 52,311 | Paris 104.49 102.14 97-5 102.97 Boulenger|] 6,006 | Calais 109.98 107.92 101.63 107.9 Noirot 4,000 | Dijon 99.7 116.0 103.5 Breslau 8,084 | Ziirich 103.9 103.1 117.6 186.6 Stieda TOO, 590 Alsace- I05.0 108 106.27 | Contradict : 259° | Lorraine 5:03 “39 227, oye Berner 267,946 | Sweden 104.61 106.23 107.45 to6.0 | Contradictory (see text). The above table (in its upper part taken mainly from Hensen, after CEsterlen) shows vividly how much the results of Stieda and Berner conflict with the law of Hofacker and Sadler. In regard to Berner’s statistics, it ought to be further noted that the figures quoted refer to cases where the father or mother is only from I to 10 years the older. If the father be more than ten years older, the male majority is 103.54; if the mother be more than ten years older, the proportion is 104.10 again, against Hofacker’s and Sadler’s conclusion. Compared with the above human statistics, Schlechter’s results in regard to horses also militate against the alleged law. In regard to plants, various naturalists have drawn attention to the influence of age upon sex. The following observations are quoted by 36 THE EVOLUTION OF SEX. Heyer :—In Leontarus domestica, according to Rumpf, the female plant may bear male blossoms before its proper female flowers. In Morus nigra, and in other cases, according to Miller, male flowers may be borne first, and afterwards fruit. Treviranus observed that the first flowers of beech, chestnut, and other trees are male. Clausen gives similar examples ; and Hoffmann notes that in the horse-chestnut, and several other cases, male flowers appear first, and afterwards hermaphrodites or females. Most of the results in regard to the influence of age are, however, extremely unsatisfactory and conflicting. This is evident from the above statistics. The law of Hofacker and Sadler cannot be regarded as in any sense established. In fact, as Hensen remarks, unless statistics are enormously large they prove very little The number of other factors besides parental age which may operate in any case is evidently great, —health, nutrition, frequency of sexual intercourse, abstinence after the birth of a male, and the like, all reduce the feasibility of the statistical method. At present, at any rate, we are not justified in ascribing much importance to the relative age of the parent except as a secondary factor, influential doubtless in relation to nutrition. § 7. Comparative Vigour.—The best known, and probably still most influential, theory is that of ‘‘ comparative vigour.” As elaborated by Girou and others, this hypothesis connects the sex of the offspring with that of the more vigorous parent. It cannot be said, however, that facts bear out the case. Thus consumptive mothers produce a great excess of daughters, while Girou’s theory would lead us to expect the opposite. We require in fact to have “‘ vigour” analysed out into its component factors, and in so doing we shall afterwards find not only facts but reasons in favour of the conclusion, in part included in the above theory, that highly nourished females tend to produce female offspring. That form of the hypothesis which refers the determination of sex to ‘‘ genital superiority,” or to “relative ardency,” can hardly be seriously considered. In this connection it has been maintained that in “ marriages of love,” after a short betrothal, female offspring predominate ; and a number of other interesting facts of a like nature are suggested. Some scepticism as to the practicability of such inductions is, however, inevitable. § 8. Starkweathers Law of Sex.—Closely allied to the theory of comparative vigour is that elaborately worked out by Starkweather, which is suggestive enough to deserve separate summary. He starts from a discussion of the alleged superiority THE DETERMINATION OF SEX. 37 of either sex. Few maintain that the sexes are essentially equal, still fewer that the females excel; the general bias of authority has been in favour of the males. From the earliest ages philosophers have contended that woman is but an undeveloped man; Darwin’s theory of sexual selection pre- supposes a superiority and an entail in the male line; for Spencer, the development of woman is early arrested by pro- creative functions. In short, Darwin’s man is as it were an evolved woman, and Spencer’s woman an arrested man. This notion of the superiority of males has formed the basis of many theories of sex. As a good illustration of this opinion, a few sentences may be quoted from Richarz :—‘‘'The sex is not a quality transmitted from the parents, but has its basis in the degree of organisation attained by the offspring. The male sex represents to a certain extent a higher grade of organisation or development in the embryo. This is attained when the reproductive efficiency of the mother is specially well developed, and the resulting male offspring more or less resembles the mother. But if the maternal reproductive power be weak, the ovum does not attain to maleness, and the resulting female offspring more or less resembles the father.” Thus Hough thinks males are born when the maternal system is at its best ; more females at periods of growth, reparation, or disease. Tiedman and others regard female offspring as arrested in the original state; while Velpau conversely regards females as degenerate from primitive maleness. Reacting from such speculations as to superiority of either sex, Starkweather firmly maintains that ‘neither sex is physically the superior, but both are essentially equal in a physiological sense.” This is true in the average, but yet in each pair a greater or less degree of superiority on one side or other must usually be conceded. Granting this, Starkweather states, as his chief conclusion, ‘‘that sex is determined by the superior parent, also that the superior parent produces the opposite sex.” Referring the reader to the Ency. Brit. Article “SEX,” for some critical notes, it is enough here to notice, that just like “‘comparative vigour,” so “‘superiority ” has little more than verbal simplicity to recommend it, since it lumps a great variety of factors under a common name. Yet, in justice to its author, we may admit that it is the algebraic sum of these which he aims at expressing. § 9. Darwin’s Position.—Neither in regard to the origin of 38 THE EVOLUTION OF SEX. sex, nor its determination in individual cases, did Darwin see further than his contemporaries. He refers to the current theories of the influence of age, period of impregnation, and the like; and further contributes a great body of statistics on the numerical proportions of the sexes, and the supposed influence of polygamy. ‘There is reason,” he says, “to suspect that in some cases man has by selection indirectly influenced his own sex-producing powers.” He falls back upon the unanalysed “belief that the tendency to produce either sex would be inherited like almost every other peculiarity, for instance, that of producing twins.” ‘‘In no case, as far as we can see, would an inherited tendency to produce both sexes in equal numbers, or to produce one sex in excess, be a direct advantage or disadvantage to certain individuals more than to others; . . . and therefore a tendency of this kind could not be gained through natural selection.” “I formerly thought that when a tendency to produce the two sexes in equal numbers was advantageous to the species, it would follow from natural selection, but I now see that the whole problem is so intricate that it is safer to leave its solution for the future.” Any other hints that Darwin threw out, have been so well elaborated by Diising’s work on the advantageous self-regula- tion of the sex-proportions, that reference to the latter is more profitable. S 10. Diising on the Proportions of the Sexes, and the Regulation of these. —In an important work, Dusing has recently treated the whole subject with some synthetic result. He recognises that the fates or factors determining the sex are manifold, and operate at different periods. Much is determined by the condition of the reproductive elements, z.¢., by the con- stitution and habits of the parents; much depends also on the period of fertilisation ; while again the nutrition of the embryo may be of moment. Diising has collected a great body of facts, from both plants and animals, in favour of his conclusions; but the copious summary of his work, given in the article “SEX” already referred to, need not here be repeated, while some of his experimental results will be included in the next chapter. Diising’s memoir is very important, however, for this special reason, that he analyses what may be termed the mechanism by which the proportion of the sexes is regulated. Instead of vaguely referring the whole matter to natural selection, he shows in detail how the numbers are in a sense self-regulating, THE DETERMINATION OF SEX. 39 how there is always produced a majority of the sex that is wanted. ‘That is to say, if one sex be in the decided minority, or under conditions which come to the same thing, then a majority of that sex will be produced. If there be, for instance, a great majority of males, there is the greater likelihood of the ova being fertilised early, but that means a prcbable pre- ponderance of female offspring, and thus the balance is restored. It would be rash to say that in every case he makes out his contention, but his general argument, that disturbances in the proportion of the sexes bring about their own compensa- tion, is carefully and convincingly worked out. § 11. Sex of Twins.—It sometimes happens among many different classes of animals that from one ovum two organisms develop. We have then a case of ‘‘ true” twins, as opposed to cases where multiple offspring do not arise from one ovum. Such “true” twins seem to occur not uncommonly in the human species, and are either most markedly similar to one another or strongly dissimilar. The import of this is one of the minor problems of heredity, and cannot be here discussed, but we have to note the general fact, which holds without exception in the human species, that ‘‘true” twins are of the same sex. From a very early date an exception to this rule has been known in regard to cattle, and applies to some other organisms as well. From the careful researches of Spiegelberg and others, it appears that in cattle (a) the twins may be both female and then both normal, or (@) that the sexes may be different and normal, or (c) that both may be males, in which case one always exhibits the peculiar abnormality known as a ‘‘ free-martin.” The internal organs are male, but the external accessory organs are female, and there are also rudimentary female ducts. No theory has yet explained the facts of this case. : It is now necessary, with Dusing for transition, to pass from the historical mode of treatment to something more con- structive. Leaving mere hypotheses behind, as well as theories based on insufficient statistics, an induction from experimental evidence will be built up in the following chapter. 40 THE EVOLUTION OF SEX. SUMMARY. 1. The epoch at which the sex is finally determined is variable in different animals, and diverse factors operate at successive epochs. 2. Theological and metaphysical theories of sex have preceded the scientific ; observation and statistics have been resorted to before experi- ment ; and over 500 theories in all have been set forth. 3-6. That there are two kinds of ova is still for the most part an assump- tion; that the entrance of more than one spermatozoon normally occurs, and is a determining factor, is erroneous. Thury’s emphasis on the age of the ovum when fertilised is probably justified ; while Hensen extends this notion to the male element as well. The age ofthe parents is probably only of secondary import, and the law of Hofacker and Sadler is not confirmed. 7, 8. Theories of ‘‘ comparative vigour” and the like must be dismissed 5 while Starkweather’s theory of the relative superiority of either sex, and of the influence of this on the sex of the offspring, requires further analysis. 9, 10. Darwin’s position contains nothing novel, and has been superseded by Diising’s synthetic treatment and explanation of the self-regulating numerical proportion of the sexes. 10. From this point, after a note on the similar sex of ‘‘ true” twins, we pass to the experimental data and constructive treatment. LITERATURE. BERNER. —Hj. Om Kjonsdannelsens Adarsager, En biologisk Studie (with numerous references). Christiania, 1883. DARWIN, C.—The Descent of Man, Chap. VIII. London, 1871. The Variation of Animals and Plants under Domestication. Lond. Dusinc, C.—Die Regulierung des Geschlechtsverhaltnisses bei der Vermehrung der Menschen, Thiere, und Pflanzen. Jena, 1884; or, Jen. Zeitsch. f. Naturw., XVII., 1883. GEDDES, P.—As before. HENSEN, V.—Physiologie der Zeugung. Hermann’s Handbuch der Physiologie, Bd. VI., pp. 304, with references to Ploss, Schultze, &c. Leipzig, 1881. His, W.—Theorien der geschlechtlichen Zeugung. Arch. f. Anthropologie. Bde. IV.-VI. HoOFACKER.—Ueber die Eigenschaften, welche sich bei Menschen und Thieren auf die Nachkommen vererben. Tiibingen, 1828. LAULANIE, F.—Comptes Rendus, CI., pp. 593-5. 1885. RoLpeH, W. H.—As before. RotuH, E.—Die Thatsachen der Vererbung (historical). Berlin, 1885. PFLUGER, E.—Ueber die das Geschlecht bestimmenden Ursachen und die Geschlechts-verhaltnisse der Frosche. Arch. f. d. ges. Physiol. XXIX. SADLER.—The Law of Population. London, 1830. SCHLECHTER.—Ueber die Ursachen welche das Geschlecht bestimmen. Rev. f. Tierheilkunde. Wien, 1884. Biol. Centralblt., [V., pp. 627-9. STARKWEATHER.—The Law of Sex. London, 1883. STIEDA.—Das Sexual Verhaltniss bei Geborenen. Strasburg, 1875. SUTTON, J. B.—General Pathology. London, 1886. THURY.— Ueber das Gesetz der Erzeugung der Geschlechter. Leipzig, 1863. Warpeus.—Allgemeine Bevolkerungs-Statistik. Leipzig, 1861. (CIsUAUP Ea es JO THE DETERMINATION OF SEX. (Experiment and Rationale.) § 1. Lnfluence of Nutrition.—Throughout nature the influence of food is undoubtedly one of the most important environ- mental factors. "To Claude Bernard, indeed, the whole problem of evolution was very much a question of variations of nutrition. ‘‘ L’evolution, c’est ’ensemble constant de ces alternatives de la nutrition; c’est la nutrition considerée dans sa réalité, em- brassée d’un coup d’ceil a travers le temps.” It is fitting that we should begin our survey of the factors known to influence sex with the fundamental function of nutrition. (a) Zhe Case of Tadpoles—Not a few investigators who have passed from statistics and hypothesis to experiment and induction, have found their material in tadpoles, where the sex seems to remain for a comparatively long period indeterminate. If we take the verdict of Yung, who has had most experience with these forms, tadpoles pass through a hermaphrodite stage, in common, according to other authorities, with most animals. During this phase external influences, and especially food, decide their fate as regards sex, though the hermaphroditism, as we shall afterwards see, sometimes persists in adult life. It is fair, however, to notice that Pfliger gives a somewhat different account of the actual facts, distinguishing among tadpoles three varieties—(qa) distinct males, (4) distinct females, and (c) herma- phrodites. In the last, testes, or male organs, develop round primitive ovaries, and if the tadpoles are to become males the enclosed female organs are absorbed. Adopting the view stated by Yung, we shall simply state the striking results of one series of observations. When the tadpoles were left to themselves, the percentage of females was rather in the majority. In three lots, the proportions of females to males were as follows :—54 : 46; 61:39; and 56:44. The average number of females was thus about 57 in the hundred. 42 THE EVOLUTION OF SEX. In the first brood, by feeding one set with beef, Yung raised the percentage of females from 54 to 78; in the second, with fish, the percentage rose from 61 to 81; while in the third set, when the especially nutritious flesh of frogs was supplied, the per- centage rose from 56 to 92. ‘That is to say, in the last case the result of high feeding was that there were 92 females to 8 males. From the experience and carefulness of the observer, these striking results are entitled to great weight. (6) Case of Bees—The three kinds of inmates in a beehive are known to every one as queens, workers, and drones ; or, as fertile females, imperfect females, and males. What are the factors determining the differences between these three forms? In the first place, it is believed that the eggs which give rise to drones are not fertilised, while those that develop into queens and workers have the normal history. But what fate rules the destiny of the two latter, determining whether a given ovum The Queen (A), Worker (c), and Drone (8) of the Common Hive-Bee. will turn out the possible mother of a new generation, or remain at the lower level of a non-fertile working female? It seems certain that the fate mainly lies in the quantity and quality of thefood. Royal diet, and plenty of it, develops the reproductive organs of the future queens ; sparser and plainer food retards the sexuality of the future workers, in which reproductive organs do not develop. Up to a certain point, the nurse bees can THE DETERMINATION OF SEX. 43 determine the future destiny of their charge by changing the diet, and this in some cases is certainly done. If a Cea on the way to become a worker receive by chance some crumbs from the royal superfluity, the reproductive function may develop, and what are called “fertile workers,” to a certain degree above the average abortiveness, result; or, by direct intention, a worker grub may be reared into a queen bee. The following table, after a recent analysis by A. von Planta, shows the differences of diet as far as solids are concerned. cent., for drones 72.75 per cent., For queens 69.38 per and for workers 71.63 per cent. is water. SOLIDS. Queens = nee attr diss, | Wor Workers. | Nitrogenous 45-14 55-91 31.67 51-21 ae Fatty TSI II.9gO 4.74 6.84 Glucose .. 20.39 9-57 38.49 27.65 Ashes 4.06 2.02 | From the above, it is seen that the queen larvze get a quantity of fatty material double that given to the workers. The drones at first receive a large percentage of nitrogenous material, but this soon falls below the share which workers and queens obtain. The fatty material, at first large, soon falls to about a third of that given to the queens. Hence the percentage of glucose, except at first, is so much larger than in the other two cases. It is not necessary, however, to go into details to see the importance of the main point, that differences of nutrition, in great part at least, determine the all-important distinctions between the development and retardation of femaleness. Nor are there many facts more significant than this simple and well- known one, that within the first eight days of larval life, the addition of a little food will determine the striking structural and functional differences between worker and queen. Eimer has drawn attention to the interesting correlation ex- hibited in the fact that a larva destined to become a worker, but converted into a queen, attains with the increased sexuality all the little structural and psychological differences which otherwise distinguish a queen. Regarding fertilisation as a sort of nutrition, he considers drones, workers, and queens as three terms of a series, and the same view is ‘suggested by Rolph. Eimer recalls some interesting corroborations from humble bees. There the queen mother, awakened from her winter sleep by the spring sun, makes a nest, collects food, and lays her first 4A THE EVOLUTION OF SEX. brood. These are not too abundantly supplied with nourish- ment, the queen having much upon her shoulders ; they develop into small females, workers in a sense, but yet fertile, though only to the extent of producing drones. By-and-by a second brood of workers is born; these have the advantage of the existence of elder sisters, are more abundantly nourished, and develop into large females. Still, like the first brood, they pro- duce drones, though occasionally females. Finally, with the advantage of two previous broods of small and large females, the future queens are born. ‘The above facts not only afford an interesting corroboration of the influence of nutrition upon sexuality, but are of importance as suggesting the origin of the more highly specialised society of the hive bee. (c) Vow Stebold’s Experiments.—With a somewhat different purpose than that at present pursued, Von Siebold made a series of careful observa- tions on a species of wasp, WWematus ventricosus. ‘These afford, as Rolph has noted, some valuable results in regard to the determination of sex. In this wasp, the fertilised ova, unlike those of hive bees, develop into males as well as females; while the unfertilised, or parthenogenetic eggs, may pro- duce females in small percentage. From spring onwards, as warmth and food both increased, Von Siebold estimated the percentages of males and females in broods of larvz reared from fertilised ova. The results of a series of observations may be condensed in a table :-— | | END OF LARVAL PERIOD | Percentage wl ie? |) esa (Pupation). | Females. Females. Males. 15th June hp =o it I4 19 | 136 July ate oe 52 77 66 66 July Ei wie cate 269 579 215 August .. me = 340 : 25 End of August Seer 500 September ae ae 100 As Rolph remarks, the results are not altogether satisfactory for the present purpose, “‘ but this much is clear, that the percentage of females in- creases from spring to August, and then diminishes. We may conclude without scruple, that the production of females from fertilised ova increases with the temperature and with the food supply (Asszmzlationsletstung), and decreases as these diminish.” From the work of Rolph, which is full of a suggestiveness which the author unfortunately did not live to elaborate, we shall quote another paragraph summing up further experiments of Von Siebold :— ‘* Not less instructive,” he says, ‘‘are the experiments with unfertilised ova (see Table). “This table shows the same general result as before. The more abundant the metabolism (Stoffwechse/) and the nutrition, the greater THE DETERMINATION OF SEX. 45 tendency to the production of females, which at the beginning and at the end are wholly absent. In the above series of experiments, they only appear when the metabolism and the nutrition were so abundant that the entire development of the young wasps only occupied eighteen or | No. of Duration of Embryonic Sex Experiments. | and Larval State. 5 II 21 days All Males. 12 TOM 56 All Males. 13 gts}. op 493 Males. 2 Females. 14 Th tee 265 5 2 ” 15 Eg: 374 8 » 16 sts}! sige THOfs) ee I we 17 24a is Tie oe | fewer days up to the period of pupation.” The peculiarity in this last case, if the experiments were correct, is that in parthenogenesis, where the production of males is the normal condition, favourable environmental influences anvear to introduce females. Two Forms of a Common Plant-Louse or Aphis.— This figure equally well illustrates three different things,—a winged male and a wingless female; a winged and a wingless parthenogenetic female; a winged sexual female and an ordinary wingless parthenogenetic female.—From Kessler. (2) Case of Aphides—One of the most familiar illustrations of the influence of nutrition upon sex, is found in the history of the plant-lice or aphides, which is indeed full of other suggestions in regard to the whole theory of sex and reproduc- tion. Details in regard to these plant-lice, which multiply so rapidly upon our rose-bushes, fruit-trees, and the like, differ 46 THE EVOLUTION OF SEX. somewhat in the various species, but the general facts are re- cognised to be as follows. During the summer months, with favourable temperature and abundant food, the aphides produce parthenogenetically generation after generation of females. The advent of autumn, however, with its attendant cold and scarcity of food, brings about the birth of males, and the consequent recurrence of strictly sexual reproduction. In the artificial ~ environment of a greenhouse, equivalent to a perpetual summer of warmth and abundant food, the parthenogenetic succession of females has been experimentally observed for four years,—it seems in fact to continue until lowering of the temperature and diminution of the food at once re-introduce males and sexual reproduction. (ce) Butterflies and Moths.—Still keeping to insects, we may note Mrs Treat’s interesting experiment, that if caterpillars were shut up and starved before entering the chrysalis state the resultant butterflies or moths were males, while others of the same brood highly nourished came out females. Gentry too has shown for moths, that innutritious or diseased food produced males, and suggests this as a partial explanation of the excess of male insects in autumn, although we suspect that tempera- ture is in this instance probably more important. (f) Crustaceans.—In support of the same contention, Rolph has drawn attention to the following among other facts. One of the brine shrimps (Artemia salina) resembles not a few crustaceans in the local and periodic scarcity or absence of males, associated of course with parthenogenesis. At Mar- seilles, Rolph says, this artemia lives in especially favourable conditions, as its large size plainly indicates ; there it produces only females. Where the conditions of existence are less prosperous, it produces males as well. “A certain maximum of abundance and optimum of vital conditions in partheno- genetic animals—daphnids and aphides, Apus, Branchipus, Artemia, and numerous other crustaceans—produce females ; while less favourable conditions are associated with the produc- tion of males.” In regard, however, to water-fleas (daphnids), it is fair to notice that Rolph’s conclusions do not quite consist with Weismann’s, who, with unique experience in regard to these curious little animals, is disinclined to allow the direct influence of temperature and nutrition in the matter. (¢) Mammals.—When we pass to higher animals, the diffi- culties of proving the influence of nutrition upon sex are much THE DETERMINATION OF SEX, AZT greater. Yet there are decisive observations which go to increase the cumulative evidence. ‘Thus an important experiment was long ago made by Girou, who divided a flock of three hundred ewes into equal parts, of which the one-half were extremely well fed and served by two young rams, while the others were served by two mature rams and kept poorly fed. ‘The propor- tion of ewe lambs in the two cases was respectively sixty and forty per cent. In spite of the combination of two factors, the experiment is certainly a cogent one. Dusing brings forward further evidence in favour of the same conclusion, noting, for instance, that it is usually the heavier ewes which bring forth ewe lambs. He emphasises the fact, that the females having a more serious reproductive sacrifice, are more dependent on variations of nutrition than males. Even in birds, as Stolzmann points out, there is a much greater flow of blood to the ovaries than to testes,—the demands are greater, and the consequences therefore more serious if these are not fulfilled. (Z) In the human species, lastly, the influence of nutrition, though hard to estimate, is more than hinted at. Ploss may be mentioned as an authority who has emphasised this factor in homo. Statistics seem to show, that after an epidemic or a war the male births are in a greater majority than is usually the case. Dusing also points out that females with small placenta and little menstruation bear more boys, and contends that the number of males varies with the harvests and prices. In towns, and in prosperous families, there seem to be more females, while males are more numerous in the country and among the poor. (¢) Determination of Sex in Flants.—It is at present ex- tremely difficult to come to any very satisfactory conclusion in regard to the influence of nutrition upon the sex of plants. The whole subject, as far as its literature is concerned, has been recently discussed by Heyer, but his survey is by no means a sanguine one. His conclusions, in fact, seem to land him in a scepticism as to all modification of the organism by environmental influences, which we should of course be far from sharing. It must be admitted that the experiments of Girou (1823), Haberlandt (1869), and others, yielded no cer- tain result; while the conclusions of some others, are conflicting enough to justify not indeed Heyer’s despair, but his present caution. Still a few investigations, especially those of Meehan (1878), which are essentially corroborated by Diising (1883), 48 THE EVOLUTION OF SEX. go to show, for some cases, that abundant moisture and nourish- ment do tend to produce females. Some of Meehan’s points are extremely instructive. Thus old branches of conifers, overgrown and shaded by younger ones, produce only male ‘inflorescence. Various botanists, quoted by Heyer, confirm one another in the observation, that prothallia of ferns grown in unfavourable nutritive conditions produce only antheridia (male organs), and no archegonia or female organs. The botanical evidence, though by no means very strong, certainly corroborates the general result that good nourishment produces a preponderance of females. The contrast of the sexes in our common dizcious plants is here very instructive. Taking for instance the dog-mercury (Alercurialis perennis) of any shady dell, or the day lychnis (Z. dzurna), so often hardly less abundant on its sunnier slopes, experiments are still certainly wanting with regard to given plants, as to what cir- Male and Female Flowers of Pink Campion (Lychuis diuvna). cumstances originally determined their sexual differences ; but the fact of superior constitutional vegetativeness in the females is here so peculiarly obvious, that it can hardly fail to arouse a strong impression, that more or less advantageously nutritive conditions, whether of the embryo or of the seedling, are suffi- cient to account for the differences of sex. § 2. Influence of Temperature.—In this connection not a few writers have referred to an observation by Knight, which, from its comparatively ancient date, perhaps deserves to be recorded in his own words, if only to show the necessity of THE DETERMINATION OF SEX. AQ caution in such matters. A water-melon was grown in a heated glass-house, where the temperature sometimes rose on warm days to 110° Fahr. “The plant grew with equal health and luxuriance, and afforded a most abundant blossom ; but all its flowers were male. ‘This result did not in any degree surprise me, for I had many years previously succeeded, by long con- tinued very low temperature, in making cucumber ?! ants produce female flowers only; and I entertain but little doubt that the same fruit stalks might be made, in this and the preceding species, to support either male or female flowers in obedience to external causes.” This experiment was obviously more sanguine than satis- factory. Heyer justly points out that of the water-melon only a single plant was taken. Furthermore, he says, the water- ‘melon in nature usually bears only female flowers on the aur of the older twigs, and may bear only a minimum number of these. Knight’s observations on cucumbers are also open to serious objections, and were too scanty to prove anything. Meehan finds that the male plants of hazel grow more actively in heat than the female; and Ascherson fas made the interesting observation, that the water-soldier (S¢ratiotes aloides) bears only female flowers north of 52° lat., and from 50° south- wards only male ones. In the human species, Dusing and others have noted that more males are born during the colder months; and Schlechter has reached the same results from observations upon horses. The temperature of the time, not of birth but of sex determina- tion, must of course be noted; nor must it be forgotten that temperature may have its influence indirectly through the nutritive functions. § 3. Summary of Factors.—If we now sum up ae case, it must first be recognised that a number of factors co-operate in the determination of sex; but that the most important of these, with increasing penetration of analysis, may be more and more resolved into plus or minus nutrition, operating upon parent, sex elements, embryo, and in some cases larvee (2) Starting with the parent organisms themselves, we find this general conclusion most probable,—that adverse circum- stances, especially of nutrition, but also including age and the like, tend to the production of males, the reverse conditions favouring females. (2) As to the reproductive elements, a highly nourished D 5° THE EVOLUTION OF SEX. ovum, compared with one less favourably conditioned, in every probability will tend to a female rather than to a male develop- ment. Fertilisation, when the ovum is fresh and vigorous, before waste has begun to set in, will corroborate the same tendency. (c) Then if we accept Sutton’s opinion as to a transitory hermaphrodite period in most animals, from which the transition to unisexuality is effected by the hypertrophy of the female side or preponderance of the male in respective cases, the vast importance of early environmental influences must be allowed. The longer the period of sexual indifference (though this term be an objectionable one) continues, the more important must be those outside factors, whether directly operative or indirectly through the parent. Here again, then, favourable conditions of nutrition, temperature, and the like, tend towards the pro- duction of females, the reverse increase the probability of male preponderance. The general conclusion, then, more or less clearly grasped by numerous investigators, is that favourable nutritive con- ditions tend to produce females, and unfavourable conditions males. S 4. Let us express this, however, in more precise language. Such conditions as deficient or abnormal food, high temperature, deficient light, moisture, and the like, are obviously such as would tend to induce a preponderance of waste over repair,— a katabolic habit of body,—and these conditions tend to result in the production of males. Similarly, the opposed set of factors, such as abundant and rich nutrition, abundant light and moisture, favour constructive processes, z.é., make for an anabolic habit, and these conditions result in the production of Jemales. With some element of uncertainty, we may also include the influence of the age and physiological prime of either sex, and of the period of fertilisation. But the general conclusion is tolerably secure,—that in the determination of sex, influences inducing katabolism tend to result in production of males, as those favouring anabolism similarly increase the probability of females. § 5. This is not all, however; the above conclusion is indeed valuable, but it acquires a deeper significance when we take it in connection with the result of a previous chapter. There it was seen, as the conclusion of an independent induction, that the males were forms of smaller size, more active habit, higher THE DETERMINATION OF SEX, 51 temperature, shorter life, &c.; and that the females were the larger, more passive, vegetative, and conservative forms. Theories of “inherent” maleness or femaleness were rejected, since practically merely verbal; more accurately, however, they have been interpreted and replaced by a more material conception, which finds the bias of the whole life, the resultant of its total activities, to be a predominance of the protoplasmic processes either on the side of disruption or construction. ‘This conclusion has still to receive cumulative proof, but one large piece of evidence is now forthcoming, that, namely, of the present chapter. If influences favouring kata- bolism make for the production of males, and if anabolic conditions favour females, then we are strengthened in our previous conclusion, that the male is the outcome of pre- dominant katabolism, and the female of equally emphatic anabolism. § 6. Wetsmann’s Theory of Heredity.—In thinking of the environment as a factor determining the sex, it is impossible to ignore that such facts as we have noted above have some bearing upon the problem of heredity. Much of the recent progress in the elucidation of the facts of inheritance has been due to Weismann, who, in his theory of the continuity of the germ-plasma, has restated the very important and fundamental conception of a continuity between the reproductive elements of one generation and those of the next. To this restatement we shall afterwards have to refer; it is with another position, not peculiar to, but emphasized by the same authority, that we have here to do, viz., with his denial of the inheritance of individually acquired characters. Any new character exhibited by an organism may arise in one of two ways, which it is easy enough to distinguish theoretically ;—it may be an outcrop of some property inherent in the fertilised egg-cell, that is, it may have a constitutional or germinal origin; but, on the other hand, it may be impressed upon the individual organism by the environment, or acquired in the course of its functioning, that is, it may have a functional or environmental origin. Thus an increase of calcareous matter in an animal might well be wholly of constitutional origin; but a change to a new diet, or toa new medium, might be followed by modifications arising, in one sense, from without. But all such functional and environ- mental variations are, according to Weismann, restricted to the individual organism ; they are not transmissible. 52 THE EVOLUTION OF SEX. And why not? This denial of the inheritance of dints from Without, and of acquired habits other than constitutional, can be no mere optimism on Weismann’s part. It is, he maintains, a scientific scepticism, based on the one hand on the absence of data demonstrating what we may still call the current belief, and on the other hand on the improbability of changes pro- duced as above explained reacting from the “body” on the reproductive cells. Ifsuch areaction do not occur, Weismann’s position is secure; and though in a system saturated with alcohol, or transferred to a new climate, the reproductive cells may vary along with the body, no modification of nerve or muscle can, as such, be transmitted in inheritance. In short, the reproductive protoplasm must be in a sense insulated, and leads a charmed life away from external disturbance. This view, supported as it is by many authorites, is obviously of the utmost importance, both for the general theory of evolu- tion, and for such practical problems as those of the pathologist and the teacher. Its full consideration is here impossible, involving matter enough for a special treatise on heredity. The difficulty of any yea or nay lies in the relative scarcity of experimental data, in the divergence of opinion as to the pathological evidence, and very largely in the difficulty of applying our logical or anatomical distinctions to the intricate facts of nature. Thus the distinction between ‘‘ acquired,” and germinal or constitutional, is easily made on paper, but is difficult in actual practice ; nor is the line between a variation of the reproductive cells, along with the body, and one produced by the body, readily drawn in concrete cases. One criticism is suggested by the present chapter. The assumed insulation or separateness of the reproductive elements from the general life of the body, how far is this real? In view of the genuine unity of the organism, a charmed life of one of the systems seems to some a “‘veritable physiological miracle;” and therefore we point to such a case as Yung’s tadpoles, where an outside influence of nutrition saturated through the organism and did affect the reproductive elements, not indeed to the degree of altering any structural feature of the species, but yet to the extent of altering the natural numerical proportions of the sexes. THE DETERMINATION OF SEX. 53 g SUMMARY. 1. Nutrition is one of the most important factors in determining sex. In illustration, note (a) the experiments of Yung, which raised the percentage of females from 56 to 92 by good feeding ; (4) the case of bees, where the differences between queen and worker well illustrate the enormous results of a slight nutritive advantage; also the case of humble-bees, with three successive broods increasing in nutritive prosperity and in femaleness ; (c) Von Siebold’s experiments with a wasp, which showed most females in favourable conditions; (@) Aphides, in prosperity of summer, yield a succession of parthenogenetic females, in cold and scarcity of autumn males return ; (¢) starved caterpillars of moths and butterflies become males ; (/) Rolph’s observations on crustaceans; () also the facts noted by Girou, Diising, and others, on the influence of good nourishment of mammalian mothers in favouring female offspring; (4) the hints of the same results in the human species ; (z) the various observations in regard to plants which favour the same general conclusion. 2. As to the influence of temperature, favourable conditions again tend to femaleness of offspring, extremes to males. 3. These factors are now added up,-—(a) the nutrition, age, &c., of parents; (4) the condition of the sex elements; (c) the environment of embryo. 4. The generalisation is thus reached,—anabolic conditions favour preponderance of females, katabolic conditions tend to produce males. 5. But females have been already seen to be more anabolic, and males more katabolic. This view of sex is therefore confirmed. 6. How does Weismann explain the determination of sex, which illus- trates an outside influence penetrating to the reproductive cells ? LITERATURE. See works mentioned in Chapter III., especially those of Diising, Geddes (article Sex, Ency. Brit.), Hensen, and Sutton; also those of Eimer, Geddes, and Rolph in Chapter II. DusinG, C.—As before ; also, Die experimentelle Priifung der Theorie von der Regulirung des Geschlechtsverhaltnisses. Jen. Zeitschr. f. Naturwiss. XIV., Supplement, 1885. HeEYeER, F.—Untersuchungen iiber das Verhaltniss des Geschlechtes bei einhausigen und zweihausigen Pflanzen, unter Beriicksichtigung des Geschlechtsverhaltnisses bei den Thieren und den Menschen, Ber. landwirthschaftl. Inst. Halle. V. 1884, pp. 1-152. MEEHAN, T.—Relation of Heat to the Sexes of Flowers. Proc. Acad. Nat. Science, Philadelphia (1884), pp. 111-117. SEMPER, C.—The Natural Conditions of Existence as they Affect Animal Life. Internat. Science Series, London, 1881. THomson, J. A.—Synthetic Summary of the Influence of the Environment upon the Organism. Proc. Roy. Phys. Soc. Edin., IX. (1888), pp. 446-499. (Supplementary to Semper’s work, with bibliography.) The History and Theory of Heredity. Proc. Roy. Soc. Edin., 1889, pp. 91-116, with bibliography. 54 THE EVOLUTION OF SEX. WEISMANN, A.—Die Continuitat des Keimplasmas als Grundlage einer Theorie der Vererbung, Jena, 1885; and numerous other papers, now translated, in I vol.—Essays upon Heredity and Kindred Biological Problems, authorised translation, edited by E. B. Poulton, S. Schonland, and A. E. Shipley, 8vo. Oxford, 1889. WILCKENS, M.—Untersuchungen iiber das Geschlechtsverhaltniss und die Ursachen der Geschlechtsbildung in Haustieren. Biol. Centralblt. VI. (1886), pp. 503- 510 5 Landworth, J. B., XV., pp. 607-610. Yunc, E.—Contributions 4 l’Histoire de I’ Influence des milieux Physiques sur les Etres Vivants. Arch. Zool. Exper., VII. (1878), pp. 251-282 ; (1883), pp. 31-55; Arch. Sci. Phys. Nat., XIV. (1885), pp. 502-522, &c., &c. BOOTKe elke eee ee NPY SiS Or SEA ORGANS HOS SHES, Clee ILS: (CIBUAIE TE IR We SEXUAL ORGANS AND TISSUES. T is the object of this portion of the book to continue the analysis of sexual characters, but now in a deeper way, reviewing successively the organs, tissues, and cells concerned in sexual reproduction. ‘The essential and auxiliary organs of the two sexes, the frequent combination of these in hermaphro- dite plants and animals, the sex-cells both male and female, will be discussed in order. This survey will be for the most part structural or morphological ; the special physiology of sexual union and of fertilisation will be discussed at a later stage. § 1. Lssential Sexual Organs of Animals.—It is now a well established fact that among the ciliated infusorians, which swarm especially in stagnant waters, a process occurs which cannot but be described as in part sexual reproduction. Two individuals, to all appearance alike be it noted, become tempor- arily associated, and interchange some of the elements of their accessory nuclear bodies. This process of fertilisation is essential to the continued vigour of the species, and will be afterwards described at length. Such a very simple form of sexual union differs from what occurs in higher animals, in two conspicuous respects,—(a@) the organisms are apparently quite similar in form and structure; (4) they are unicellular, and thus there is no distinction between ‘‘ body” and reproductive cells. What is fertilised by the mutual exchange in those infusorians is, roughly speaking, the entire animal, for the whole is but a unit mass of living matter. Among the protozoa, however, loose colonies of cells occur, which bridge the gulf between unicellular and multicellular animals. In these we find the first indications of the after- wards conspicuous difference between ‘‘body” and reproductive cells. From these loose colonies, certain of the units are set adrift, and meeting with others more or less like themselves fuse to form a double cell, virtually a fertilised ovum, from 58 “THE EVOLUTION OF SEX. which by continuous division a fresh colony is then developed. In these transition forms there are thus reproductive cells of slight distinctness, but as yet obviously no sexual organs. Volvox, a loose colony of cells, with some set apart for reproduction, after Kirchner, When we pass to the sponges, we find colonies consisting of myriads of cells, among which there is a considerable division of labour. An outer layer (or ectoderm) usually consisting of much subordinated cells, an inner layer (or endoderm) of predominantly active and well-nourished cells, a middle layer of heterogeneous constituents, can always be distinguished. Every average infu- sorian is as good as its neighbours, so far as reproduction of new individuals by division is concerned ; in the colonial protozoa, the units that are set adrift are very little different from their fellows that remain behind ; but this ceases to be true when we pass to colonies where considerable division of labour has been established. It is certainly true that even a tiny fragment of sponge, cut off from the larger mass, may, if it contain sufficient samples of the body, and if the conditions be favourable, repro- duce a new individual. Cultivators of bath sponges habitually take advantage of this fact. But the sponge starts its new SEXUAL ORGANS AND TISSUES. 59 colonies for itself usually in quite a different way, namely, by the process of sexual reproduction. Among the cells of the middle stratum of the sponge body certain well-nourished passive cells appear. ‘These are the ova, at first very like, but eventually well marked from the other constituent units of the layer. Besides these there are other cells, either in the same sponge or in another, which exhibit very different characters. Instead of growing large and rich in reserve material like the ege-cells or ova, they divide repeatedly into clusters of infini- tesimal cells, and form in so doing the male elements or spermatozoa. ‘The male and female cells meet one another, they form a fertilised ovum ; the result is continued division of the latter tilla new sponge is built up. Here then there are special reproductive cells, quite distinct from those of the ‘““body”; and here, furthermore, these reproductive cells are markedly contrasted as male and female elements. As yet, however, there are no sexual organs. Passing to the next class, the stinging animals or ccelenter- ates, we find in one of the simplest and most familiar of these, the common fresh-water hydra, a good illustration of primitive sexual organs. As in sponges, a cut-off fragment of the body, if sufficient samples of the different component cells are in- cluded, is able to reconstitute the whole. But no one body-cell has of course any such power; this is possible for the fertilised ovum alone. Now this ovum occurs, not anywhere within a given layer as in sponges, but always near one spot on the body. Towards the base of the tube a protuberance of outer layer cells is developed. ‘This forms a rudimentary ovary, or female organ. It has this peculiarity, not however unique, that while the organ consists of not a few cells, only one of these becomes an ovum. A similar protrusion, or more than one, often at the same time and on the same animal, may be recognised further up the tube, nearer the tentacles of the hydra. Of somewhat smaller size, such a superior protuberance consists of numerous small cells, most of which, multiplying by division, form male elements or spermatozoa. We have here the simplest possible male organ or /esfzs. More elaborate organs occur in the other ccelenterates, complicated however by two interesting facts, which will be afterwards discussed. (a) Many of the ccelenterates are well known to form elaborate colonies,—zoophytes, Portuguese men- of-war, and the like. In these, division of labour frequently 60 THE EVOLUTION OF SEX. goes further than the setting apart of special organs. Entire individuals become reproductive “ persons” (as they are techni- cally called), in contrast to the nutritive persons of the colony. (2) In some of those reproductive individuals, a curious phenomenon, known as migration of cells, has been observed by Weismann and others. The reproductive cells, arising in various parts of the body, have been shown to migrate in some cases to another part, where they find final lodgment in more or less definite organs. This occurrence is intimately associated with ‘“‘alternation of generation,” and will be afterwards discussed under that heading. It is in nowise the purpose of the present work to describe the details respecting the ovaries and testes, as they occur in the various classes of animals. It is enough for our purpose to have emphasised the fact of their gradual differentiation, and to note that they are almost always developed in association with the middle layer of the body, and usually occupy a posterior position on the wall of the body-cavity. The details will be found in any standard work on comparative anatomy, very con- veniently for example in Prof. Jeffrey Bell’s “Comparative Anatomy and Physiology,” London, 1885. § 2. Associated Ducts.—It is only in a few animals, like hydra and its allies, that the ovaries and testes are external organs, which have simply to burst to liberate their contents. They are usually of course internal, and thus arises the necessity of some means of communication with the outside world. In the simplest cases, the male elements find their way out to the sur- rounding medium without any specialised mode of exit. They there meet, by chance combined with physical attraction at short range, with the ova, which in the simplest cases again have found their way out in an equally primitive fashion. Thus in the enigmatical parasitic mesozoa (orthonectids, &c.), liberation of the germs may occur by perforation or by rupture of the excessively simple bodies. In some of the marine worms (¢.g., Polygordzus), the liberation of the ova at least is accompanied by the fatal rupture of the mother organism, a vivid instance of reproductive sacrifice. Even in some of the common nereids, the same uneconomical mode of liberation by rupture appears to occur. The forcible rupture may be referred to pressure of the relatively large mass of growing cells which the ovaries often present. As high up as back-boned animals, the absence of ducts may be traced. Thus among the sea-squirts or tunicates, the reproductive organs are fre- quently ductless, and the same thing is true of some fishes. The sex-cells burst into the body-cavity, and thence find their way to the exterior by aper- tures. In most cases, where ducts are absent, fertilisation of the ova is external, but this is not necessarily so. In sponges, for instance, fertilisa- tion is almost always internal. Male elements are washed in by the water- currents, find their way to the ova, and fertilise them zz sztu#. Almost without exception, embryo-sponges, not ova, make their way to the exterior. In the higher animals, where definite ducts are present, alike for the inward SEXUAL ORGANS AND TISSUES. 61 passage of spermatozoa and the exit of ova or embryos, it ought further to be noticed that the ovaries can hardly ever be said to be in direct connec- tion with their ducts. The ova usually burst from the ovary into the body- cavity, whence they are more or less immediately caught up by, or forced into the canals, by which they pass outwards. With the testes it is different, for if ducts be present, they are in direct connection with the organs. It is enough to state that in the great majority of cases ducts are associated with the essential organs. Those of the male serve for the exit of the spermatozoa, and may be terminally modified as intromittent organs. Those of the females serve either solely for the emission of unfertilised eggs, or for the reception of spermatozoa, and the subsequent exit of fertilised ova or growing embryos. In some worm-types, and in all vertebrates, from amphibians onwards, the reproductive ducts are also in various degrees associated with excretory functions. For an account of the origin of the ducts in higher animals, the reader must be referred to the embryological text-books of Balfour and Hertwig, or most conveniently of Haddon. Similarly for such modifications as that of the female duct into oviduct and uterus, reference must be made to the larger anatomical works of Gegenbaur and Wiedersheim, or for a briefer account to Parker’s translation and edition of Wiedersheim’s smaller text-book, and to Prof. Jeffrey Bell’s work already mentioned. S 3. Yolk-Glands.—As we shall afterwards see, the ovum is often furnished with a large quantity of nutrient material. This serves as the food-capital for the growing embryo or young larva. It is obtained in various ways,—from the vascular fluid, from the sacrifice of adjacent cells, or from special organs known as yolk-glands or vitellaria. The yolk-glands, as they occur for instance in some of the lower worms (turbellarians, flukes, tapeworms), are of some general interest. They represent, as Graff has shown, a degenerate portion of the ovary, in which the cells have become even more highly nutritive than ova. ‘The origin of the yelk-gland,” Gegenbaur says, “is probably to be found in the division of labour of a primitively very large ovary.” In more technical language, yolk-glands are hypertro- phied or hyper-anabolic portions of the ovary. Apart from this nutritive capital, the egg is often equipped with envelopes or shells of some sort, which may be furnished by special organs, or by the sacrifice of surrounding cells, or by the walls of the ducts as the eggs pass out. § 4. Organs Auxiliary to Impregnation.—In most animals in which internal fertilisation of the ova occurs, there are in both sexes special structures auxiliary to the function of impreg- nation. ‘Thus the end of the male canal is commonly modified into an intromittent tube or penis, through which the male elements flow into the female duct. In the crustaceans some of the external appendages are often modified, as in the cray- 62 THE EVOLUTION OF SEX. fish, to serve this purpose, and the same is the case with minute structures on the posterior abdomen of many insects. Some- times, as in the snail (/Ze/zx), which may be taken as an extreme type of reproductive specialisation, separate organs are present, in which the spermatozoa are compacted into masses or packets, known as spermatophores. In most cuttle-fishes, these pass from the male ducts to one of the “arms,” which thus laden is occasionally set free bodily into the mantle-cavity of the female, where it was of old mistaken for a worm, and called Hectocotylus. So in some spiders, the palps near the mouth receive the male elements, and transfer them to the female. Special storing receptacles and secreting glands are also very frequently in association with the male ducts, and there is a long lst of curious modifications utilised in the process of copulation. ‘Thus, male frogs have their swollen thumbs, and gristly fishes their “claspers,” which are modified parts of the hind limbs, and are inserted into the cloaca of the female. The common snails eject a limy dart (speculum amoris), which appears to be a preliminary excitant to copulation. So too, in the female sex, the terminations of the duct may be modified for reception of the male intromittent organ, and special receptacles may be present for storing the spermatozoa. Where a single fertilisation occurs, as in the queen bee, previous to a long-continued egg-laying period, the importance of a storing organ is obvious. As the female is usually more or less passive during copulation, the adaptations for this purpose are less numerous than in the males. It is interesting to notice, that among amphibians, where the male often takes upon him- self distinctly maternal duties, one case is known where the female seems more active than the male during copulation. § 5. Lge-Laying Organs.—Cases where the ova simply pass out into the water, or on to the land, are of course associated with the absence of any special organs. In a great many animals, however, more care is taken, and auxiliary structures are present One of the simplest of useful developments is exhibited by glands, the viscid secretion of which moors the ova, and keeps them from being set wholly adrift. In insects, where it is specially important that the eggs should be well con- cealed, or buried in conveniently nutritive material, hints of the ancestral abdominal appendages remain as “‘ ovipositors.” Throughout the series a great variety of structures occur in this connection. SEXUAL ORGANS AND TISSUES. 63 $6. Brooding and Young-Feeding Organs. — From very lowly animals onwards, structures are present which are utilised in the protection of the young in their helpless stages. The reproductive buds of some ccelenterates become true nurseries ; in one at least of the marine worms (Sfzvorbis spirillum), a tentacle serves as a brood pouch; various adaptations, such as tents of spines, or cavities in the skin, are utilised in echino- derms. The young shelter under the hard cuticle, or among the appendages of crustaceans, in the gills of bivalves, and a cuttle-fish has been seen with the eggs in its mouth. Among the higher animals, the brood-pouch of Appendicularia (one of the very lowest Chordata), the pockets of not a few fishes, the cavities on the back of the Surinam toad, the pouches of mar- supials, are only a few instances amid a crowd. Sometimes, especially in fishes and amphibians,—e.g., the sea-horse, with its breast-pouch, and Rhinoderma darwiniz, with its enlarged croaking sacs,—it 1s the male which undertakes the brooding office. When the young are born alive, the internal female ducts become developed in this connection to form uter1. The ovary appears to serve as a womb in the genus Girardinus among fishes, but it is usually the median portion of the female duct which has this function. In placental mammals, where the young are born at an advanced stage, and where the maternal sacrifice is at its maximum, the uterine adaptations become more important and complex. The organs of lactation will be afterwards discussed. 64 THE EVOLUTION OF SEX. SUMMARY. 1. The gradual differentiation of essential sexual organs in animals,— isolated cells, aggregated tissues, definite organs. 2. Associated male and female ducts for the liberation of male-elements, fertilisation, exit of ova, or birth of embryos. 3. Yolk-glands, &c., for nourishment and equipment of the ova. Vitellaria have been interpreted as degenerate ovaries. 4. Illustrations of organs auxiliary to impregnation. In the male,— penis, storing sacs, spermatophore-making organs, ‘‘ claspers.” Curiosities, such as the hectocotylus of cuttle-fishes, and the Cupid’s dart of snails. Adaptations in the female are less frequent, but storing receptacles for the male-elements are common. 5. Egg-laying organs :—frequency of ovipositors. 6. Brood-pouches and the like are widely present in most classes of animals. LITERATURE. BALFouR, F. M.-—A Treatise on Comparative Embryology. 2 vols. London, 1881. BELL, F. JEFFREY.—Comparative Anatomy and Physiology. London, 1885. CLAus, C.—Elementary Text-Book of Zoology, trans. by A. Sedgwick. 2 vols. London, 1885. GEDDEs, P.—OpP. cit. GEGENBAUR, C.-—Elements of Comparative Anatomy, trans. by Prof. Jeffrey Bell. London, 1878. Happon, A. C.—An Introduction to the Study of Embryology. London, 1887. HENSEN, V.-—Op. cit. HERTWIG, O.—Lehrbuch der Entwicklungsgeschichte des Menschen und der Wirbelthiere. Jena, 1888. HATCHETT JACKSON’s (W.) Edition of Rolleston’s Forms of Animal Life. Oxford, 1888. HuxLey, T. H.—Anatomy of Vertebrate and Invertebrate Animals. London, 1871 and 1877. SACHS, J.—Text-Book of Botany, edited by Prof. Vines. Second edition. Oxford, 1882. And similar works. Lectures on the Physiology of Plants, trans. by Prof. Marshall Ward. Cambridge, 1887. VINES, S. H.—Vegetable Reproduction (Ency. Brit.). Lectures on the Physiology of Plants. Cambridge, 1886. WIEDERSHEIM, R.—Elements of the Comparative Anatomy of Verte- brates, trans. by Prof. W. N. Parker. London, 1886. Also un- abridged work. CECAPARE RS VAL HERMAPHRODITISM. § 1. WHEN an organism combines within itself the production of both male and female elements, it is said to be bisexual or hermaphrodite. This is the case with most flowers, and with many lower animals,—such, for instance, as earthworms and snails. It is not desirable to extend the term, as is sometimes done, to cases like ciliated infusorians, where sex itself is only incipient. Undoubtedly in those Protozoa recent researches have distinguished what in loose analogy may be called male and female nuclear elements, but this primitive condition is rather a state antecedent to sex, than a union of sexes in one organism. In most phanerogams, as every one knows, male and female organs occur on different leaves (stamens and carpels) of each flower. The flower as a whole, or the entire plant, may then be called hermaphrodite. But as the male and female organs are restricted to different leaves, each leaf is by itself unisexual, when compared, for instance, with the prothallus of a fern, which bears on the same small expansion both male and female organs. - When stamens and carpels unite together, as in orchids, a more intimate hermaphroditism is obviously developed. So with animals. While the general definition of hermaph- roditism, as the union of the two sexes in one organism, is plain enough, the union is exhibited in a great variety of ways and degrees. Of these it is necessary first to take account. S 2. Embryonic Hermaphroditism.—Some animals are hermaphrodite in their young stages, but unisexual in adult life. Allusion has already been made to the case of tadpoles, where the bisexuality of youth occasionally lingers into adult life. According to some, most higher animals pass through a stage of embryonic hermaphroditism, but decisive proof of this is wanting. E 66 THE EVOLUTION OF SEX. The research of Laulanié may now be referred to at greater length. As the result of observations on the development of the reproductive organs in the higher vertebrates, and especially in birds, he seeks to establish a strict parallelism between the individual, and what he believes to have been the racial history. In the chick, he distinguishes three main stages in the development—(1) germiparity, (2) hermaphroditism, (3) differentiated unisexuality. These he regards as recapitulating the great steps of the historic evolution. (1I.) For the first period of ‘‘ germiparity,”—from the fourth to the sixth day,—the designation, sexual neutrality, or indifference, is inappropriate, since the ‘‘ cortical ovules” of the germinal epithelium have from the first the precise morphological significance of female elements or ova. In the female, they proceed bymultiplication to form the ovary ; inthe male, they degenerate. (2.) The period of hermaphroditism begins with the seventhday. Inthe male, the male ovules, from which the sperms are after- wards developed, appear in the centraltissue; but at the same time cortical or female ovules may be seen persisting. Similarly, in the developing ovary of the female, the central or medullary portion, strictly separated by a par- tition of connective tissue from the egg-forming layer, contains a large number of medullary or male ovules. (3.) This hermaphroditism is of short duration. The cortical or female ovules disappear from the testes by the eighth or ninth day; and the medullary or male ovules have by the tenth day disappeared from the ovary. In regard to mammals, Laulanié affirms, allowing some peculiarities, that the same three stages of germi- parity, hermaphroditism, and unisexuality occur. Ploss has already been referred to as another investigator who maintains the existence of embryonic hermaphroditism. Such also is the view held by Professor Sutton, who concludes that both sets of organs are equally developed up to a definite period, and emphasises the consequent necessity for the hypertrophy of one sexual rudiment over the other. Only thus can unisexuality be established. It ought perhaps to be noted, that hyper- trophy is hardly a term strictly applicable to predominance of male over female organs, since, in our contention, the whole nature of male organs or elements is the physiological reverse of abundant nutrition. § 3. Casual or Abnormal Hermaphroditism.—In many species which are normally unisexual, a casual hermaphrodite form occasionally presents itself. The embryonic equilibrium or bisexuality—one of the two must in a variable degree exist—is retained as an abnormality into adult life. Even as far up in the organic series as birds and mammals, such casual and yet true hermaphrodites occur. In most cases at least the result is sterility. Among amphibians, which abound in reproductive peculiarities, herma- phroditism exceptionally occurs, apart from the one case (see below) where it is known to be constant. The common frog, so much dissected in our laboratories, has supplied several good illustrations. Thus Marshall notes that the testes may be associated with genuine ova, or an ovary may occur on one side, and a testis with an anterior ovarian portion upon the other. Bourne gives a case of a frog with the ovary well developed on the right side, and opposite this an ovary anteriorly replaced by testis. One of the toads (Pelobates fuscus) seems to be frequently hermaphrodite, the male being furnished with a rudimentary ovary in front of the testes. A similar hermaphroditism is not at all infrequent in cod, herring, mackerel, and many other fishes; while slightly lower down in the series, it occurs in the hagfish (JZyxz7e). Sometimes a fish is male on one side, female —— HERMAPHRODITISM. 67 on the other, or male anteriorly and female posteriorly. Sir J. W. Simpson, in a learned article on the subject, has distinguished cases of true hermaphroditism according to the position of the organs, into lateral, transverse, and vertical or double. Among invertebrates the same has been occasionally observed, especially among butterflies where striking differences in the colouring of the wings on the two sides have in some cases been found to correspond to an internal co-existence of ovary and testis. The same has been observed in a lobster, and is probably commoner than the recorded cases warrant one in asserting. As low downas coelenterates, casual hermaphroditism may occur, as F. E. Schulze showed in one of the medusoids. § 4. Partial Hermaphroditism.—An organism may be said to be truly hermaphrodite when both male and female organs are present, or when, without there being separate organs, both male and female elements are produced. It is then both anatomically and physiologically hermaphro- dite, and of this, as we shall see, there are abundant illustrations among lower animals. Snail, earthworm, and leech are examples of this hermaph- roditism, in varying degrees of intimacy. But, as we have just noticed, a species normally unisexual may occasion- ally exhibit hermaphrodite individuals. In these only one of the double essential organs may be functional, or both may be sterile. Whether physiologically or not, such animals are anatomically hermaphrodite. Both kinds of essential organs are at least present. To those must now be added a further series of cases to which the term partial hermaphroditism seems most applicable. Only one kind of sexual organ, ovary or testis, is developed ; but while one sex preponderates, there are more or less emphatic hints of the other. As the reproductive organs are to be regarded as the most important, but not by any means the sole expression of the fundamental sex-differences, it is impossible to separate partial hermaphroditism by any hard and fast line from the above, and from the next set of cases (paragraphs 3 and 5). Almost all cases of partial hermaphroditism occur as exceptions, though a few are constant. In the higher animals, partial hermaphroditism is usually expressed in the nature of the reproductive ducts. In this connection the structural resemblance of the male and female organs must be once more emphasised. Even the Greeks had their vague and fanciful theories of what we now call the homology of the reproductive organs and ducts in the two sexes. Through the labours of the anatomists of Cuvier’s school, such as his fellow- worker Geoffroy St Hilaire, and yet more through more recent embryo- logical discoveries, there is now both clearness and certainty as to the main facts. The reproductive organs proper, the ducts, and the external parts, are developed upon the same plan in male and female. Thus, except in the lowest vertebrates, what serves as an oviduct in the female, is equally present in the embryo male, and persists in the adult as a more or less functionless rudiment. In the same way, what serves as the duct for the sperms (vas deferens) in the male, is equally present in the embryo female, and persists in the adult as a rudiment, or is diverted to some other pur- pose. This is a perfectly normal occurrence, dependent upon the embryo- logical history of the ducts in question. It is necessary, however, to realise both the primitive resemblance and the fundamental unity of the two sets of organs, in order to understand how partial hermaphroditism is so fre- quent, and also to distinguish it from ‘‘ spurious hermaphroditism,” where 68 THE EVOLUTION OF SEX. a merely superficial abnormality or even injury of the ducts in one sex produces a resemblance to those of the other. We have already mentioned that in the case of twin calves, two females may occur, and both are then normal ; or two normal twin calves may be born of opposite sexes; but in the third place, if both be males, one of these very generally exhibits the peculiar phenomena of what is called a ‘‘free-martin.” In the commonest form of this, partial hermaphroditism is well illustrated. The essential organs are male, but there is a rudiment- ary uterus and vagina, and the external organs are further those of a female. It isnecessary to note, that a simulation of even this partial hermaphro- ditism may result from malformation or rudimentary development of the external organs. On this subject we may quote an acknowledged authority, alike in anatomical and embryological matters. ‘‘ From the fact,” Prof. O. Hertwig remarks, ‘‘ that the external sexual organs are originally of uniform structure in the two sexes, wecan understand the fact that, in a dis- turbance of the normal development, forms arise in which it is extremely diffi- cult to decide whether we have to deal with male or female external organs. These cases, in earlier times, were falsely interpreted as hermaphroditism. They may have a double origin. Either they are referable to the fact that in the female sex the development may proceed along the same path as in the male, or to this, that in the male the normal development may come at an early stage to a standstill, and lead to the formation of structures which resemble the female parts.” In the first case, he goes on to say, there may be a simulation of a penis, and the ovaries may even be shifted so as to produce an appearance like that of the testes within their scrotal sac. In the second case, the processes of coalescence which give rise to the penis may not occur, only a rudimentary organ is formed, and there may even be an inhibition of the usual descent of the testes into their sacs. Of this superficial hermaphroditism, really not hermaphroditism at all, there are numerous cases among mammals. But there remain a large number of recorded instances, where the anatomy of the ducts was predomi- nantly that of the sex opposite to that indicated by the essential organs, and where the combination of the two sexes was also expressed in external configuration and even in habit. Amphibians again furnish some inter- esting examples. Attached to the anterior end of the testis in various species of toad (Bzfo), there is an organ known as ‘‘ Bidder’s,” which has contents like young ova. These do not, however, get past the early stages, and the organ is quite different from the more than rudimentary ovary which occurs constantly in the males of Bufo czmereus and some other species. The two may in fact occur together. In the common frog, dissectors have also recorded several cases of hermaphroditism expressed in the ducts. Lastly, it is perhaps not going too far to include here some reference to the curious ‘‘ fatty bodies”? which occur in all amphibians at the apex of the reproductive organs in both sexes. These appear to nourish the ovary and testis, especially during hybernation, and may perhaps be associated with similar lymphoid structures in fishes and reptiles. Prof. Milnes Marshall was of opinion that the fatty bodies have resulted from the degeneration of the anterior part of the reproductive gland while still in an indifferent state ; but Mr Giles has recently traced the history of these bodies, and shown them to result from the degeneration of the anterior set of excretory tubules, the pronephros. Leaving the ducts out of account, we may arrange the HERMAPHRODITISM. 69 important phenomena of hermaphroditism in amphibians in a series as follows :— (z) Embryonic hermaphroditism, demonstrated as of normal occurrence in frog tadpoles. (>) Pantallnemepirestlten { expressed in Bidder’s organ in male toads; (also expressed in variousstates of the ducts). (c) True adult hermaphroditism, Sele BORE Tae ORY: casual in frogs, &c. Well-developed ovary, rudimentary ovary or Bidder’s organ, and “‘ fatty- bodies,” may be taken as illustrating the normal and the pathological pre- ponderance of anabolic processes. Amphibians, every one will admit, are for the most part animals of distinctly sluggish habit ; the natural characteristics of the male sex may be said to be to some extent handi- capped, and curious instances are known where the more external functions of the two sexes are strangely inverted. The male obstetric frog is not alone in taking charge of the ova, and the female of one of the efts behaves in copulation like a male. The list need not be further followed ; it is enough to note the very wide occurrence of partial hermaphroditism. In many cases, however, this takes an interesting form, by expressing itself in the external characters. Forms occur in which the minor peculiarities of the two sexes,—colouring, decorations, weapons, and the like,—appear blended together, or in which the secondary sexual characters are at variance with the internal organs. In most cases, one is safe in saying that there is no true internal hermaphroditism in any degree. Arrest of matu- rity or puberty, cessation of the reproductive functions, removal or disease of the essential organs, and the like, may alter the secondary sexual characters from female towards male, or, less frequently, zzce versa. A female deer may develop a horn, ora hen a spur, and in such cases the ovaries are generally found to be diseased. ‘The prettiest cases of superficial hermaphroditism occur among insects, especially among moths and _ butterflies, where it often happens that the wings on one side are those of the male, on the other those of the female. Only the external features have been observed in most cases; but it has been shown by dissection that such superficial blending may exist along with internal unisexuality, or, in a few cases, with genuine internal hermaphroditism. A beautiful case of intimate blend- ing of superficial sex characters was lately shown to us by Mr W. de V. Kane, of Kingstown. A specimen of butterfly (Euchloe euphenoides) showed the anterior half of the fore wings and part of the hind wings with the characteristic white ground of the female, while in the posterior half of the fore wings and © 7O THE EVOLUTION OF SEX. on most of the hind wings the characteristic sulphur of the male prevailed. In other minor ways, the characteristics of the two sexes, which are well marked, were intimately blended. Similar cases are on record. § 5. Normal Adult Hermaphroditism.— This is rare among the higher animals, but common among the lower. On the threshold of the vertebrate series, we find it indeed constant among the tunicata; but above these it is only known to occur normally in two genera of fishes, and in one genus of amphibians. “A testis is constantly found imbedded in the wall of the ovary in Chrysophrys and Serranus, and the last-named fish is said to be selfimpregnating.” In some species of male toad (e.g., Bufo cinereus) a somewhat rudimentary ovary is always present in front of the testes. All other cases among vertebrates are either casual (par. 3) or partial (par. 4). Among invertebrates, true hermaphroditism is, however, of frequent normal occur- rence, ¢.g., in sponges, coelenterates, worm types, and molluscs. It is necessary to take a brief survey of some of these. (1.) Sponwges.— As already mentioned, the sex-cells of sponges start simply among the other components of the middle layer (#esog/wa) of the body. It is at least possible that in azy sponge they may develop either into ova or into sperms, or into both, within the same organism, according to nutritive and other conditions. The facts, however, are these. Many sponges are only known in a unisexual state, while others are genuinely hermaphrodite. But among the latter it is not uncommon to find (e.g., in Sycandra raphanus) that the production of one set of elements prepon- derates over the other, and thus we have hermaphrodites with a distinctly male or female bias. In other words, they are verging towards unisexuality. It does happen in fact (¢.g., in Oscarella lobularis) that a species normally hermaphrodite may exhibit unisexual forms. It is possible, of course, that in such cases one set of sexual elements may have been wholly discharged, or may even have been overlooked in observation; but there is no improbability against the supposition, that a preponderance of favourable nutritive conditions might induce a form normally hermaphrodite to become wholly female. This, as we have seen above, is what some believe to take place in the individual history of higher forms. (2.) Calenterates.—The members of this class are higher, in having the production of the sex-cells more restricted, to definite regions, tissues, organs, or even ‘‘ persons.” The highly active Ctenophores, like Beroe, are all hermaphrodite, and that very closely. On one side of the meridional branches of the alimentary canal ova arise, on the other side sperma- tozoa. Among sea-anemones and corals the hermaphrodite condition appears in a number of cases, but is sometimes obscured by the fact that the two kinds of elements are produced at different times, corresponding to different physiological rhythms in the life of the organism. The genus Corallium (the red coral of commerce) is peculiarly instructive. The whole colony may be unisexual, or one branch of the colony, or only certain HERMAPHRODITISM. 71 individuals on a branch, while genuine. hermaphroditism of individual polyps also occurs. Among hydrozoa (zoophytes, swimming-bells, jelly- fish), hermaphroditism is a rare exception, or, we may almost say, rever- sion. The common hydra, which is a somewhat degenerate type, is hermaphrodite, though at the same time individuals may be found with only ovary or only testes. /euwtheria is also hermaphrodite, and ‘‘ abor- tive ova occur in the male of Gonothyrea lovenz.”” Sometimes a colony is hermaphrodite (Dicoryne), but the stems and individuals unisexual. Some- times a stem is hermaphrodite, but the individuals unisexual (certain sertularians). Among jelly-fishes the genus Chrysaora is known to be hermaphrodite. (3.) *§ Worms.” —The condition of the sexual organs varies enormously among the diverse types lumped together under the title of ‘worms’ or ‘* Vermes.”’ In the lowly tur- bellarians, all the genera are hermaphrodite except two, but, as in many other cases, the organs do not reach maturity at the same time, the male preceding. In the related trematodes or flukes, hermaphroditism again obtains, with one exception, or perhaps two. The certain exception is the curious parasite Lz/harzia, where the male carries the female about with him in a ‘‘gynzecophoric canal,” formed of folds of skin. In the adjacent class of cestodes or tapeworms, all the members are hermaph- rodite. These three classes are doubtless re- ~ lated, but it seems plausible to connect the retention of hermaphroditism with the de- generacy of parasitism, and also with the rich, yet at the same time stimulating nutrition, which may favour the retention of double sexuality. The utility of the hermaphrodite state, if the eggs of these animals are to be fertilised and the species maintained, can hardly be doubted, but this does not explain €é the facts. It is important to notice too, that bilharzia, ye parasitic treme lous, self-fertilisation—that is, union of the eggs and PNT Se ies sperms of the same organism—has been proved called the ‘‘ gynacophoric to occur in several trematodes, and seems to canal,” —After Leuckart. be almost universal in cestodes. This may be one of the conditions of the degeneracy of these parasites, for frequent as hermaphroditism is among plants and animals, self-fertilisation is extremely rare. Hermaphroditism is rare among the free-living nemerteans, but con- stant in the semi-parasitic leeches. The only exception to separateness of the sexes among threadworms or nematodes is the very curious case of the genus Angiostomum. ere, in an organism which is anatomically a female, the reproductive organ starts with producing spermatozoa, which fertilise the subsequent ova. The animal is thus physiologically hermaphrodite, and at the same time self-impregnating. Approaching the higher annelid worms, we find the primitive Protodrz/uzs hermaphrodite ; the earthworms are constantly so, but all their marine relatives have the sexes separate. 72 THE EVOLUTION OF SEX. The genus Sagzéta, which stands by itself, is hermaphrodite ; the same condition is known as a rarity among the ancient brachiopods (Lzzgz/a), but is frequent among the colonial Polyzoa. (4.) Achinodermata.—The members of all the echinoderm class, except one brittle star (Amphiura sguamata) and one genus of holothurians (Syzapta), have the sexes separate. (5.) Avthrofods.—Among crustaceans, hermaphroditism is a rare ex- ception, though it occurs in the majority of the fixed quiescent acorn-shells and barnacles (Czrrzfedia). There it is associated with the presence of small males, which Darwin called ‘‘ complemental.” In the Cymothoidze (Isopods), we have a curious occurrence, somewhat like that of Axgzostomum above noticed. ‘‘*The sexual organ of the young animal is male, of the old, female in function.” In such cases, one must remember the antithesis between the body proper and the reproductive cells. In youth the demands of the body during growth are greater; there is no anabolic surplus to spare, all goes to increase the body. When mature size is reached, and both growth and activities lessened, there is more likelihood of anabolic preponderance in the reproductive, as opposed to the vegetative, system. Myriopods and insects have always separate sexes, excluding of course abnormal hermaphroditism among the latter. An exception among arach- nids, otherwise unisexual, is found in the degenerate water-bears or sloth- animalcules ( Zardzgrada). (6.) Alolluscs.—Most bivalves are of separate sexes, but exceptions often occur—e.g., in common species of oyster, cockle, clam, &c. In the case of the oyster, the familiar species (Ostvea edulzs) is hermaphrodite, and a neighbouring species apparently unisexual. In both cases the organs are the same, but in O. edwlzs the same intimate recesses of the reproduc- tive organ produce at one time ova, at another time sperms. The snails, or gasteropods, are divided into two great groups, according to the twisting of theirnerves. The one group (S¢veptoneura) have the sexes separate ; the members of the other series (Zuthyzeura) are hermaphrodite. The sea-butterflies, or pteropods, are hermaphrodite, but the elephant’s tooth shells (Scaphopods) are unisexual. So in cuttle-fishes (Cephalo- pods), the sexes are separate. § 6. Degrees of Normal Hermaphroditism.—F¥rom what has been already said, it is evident that hermaphroditism may be more or less intimate. Asan entire plant, an Arum is herma- phrodite, with female flowers on the better nourished lower portion, and male flowers above. This may be paralleled by the red coral, which is sometimes female as regards one branch, and male as regards another. If we keep, however, to herma- phrodite individuals, it is evident that an orchid, with stamens and carpels united, is more closely hermaphrodite than a butter- cup flower. So in a leech, with the ovaries far forward, and independent of the long row of testes, the hermaphroditism is less intimate than in a tunicate, where the testes and ovary may form one mass, the male cells spreading over the surface of the ovary. In the same way, the organ of a scallop, which exhibits HERMAPHRODITISM. 73 more or less distinct male and female portions, is in a state of less intimate anatomical hermaphroditism than the oyster, where the same czeca of the same organ fulfil both functions a7 different times. This last caution must be kept in view throughout. If the hermaphroditism be very intimate,—that is, if the seats of the ovum- and sperm-production be very close to one another,—1t is not to be expected that the development of the two kinds of cells will go on simultaneously. Such would, indeed, be a phy- siological impossibility. Antagonistic protoplasmic rhythms may rapidly alternate, but cannot co-exist. Whether the herma- phroditism be anatomically intimate or no, there is throughout, in varying degrees, a tendency to periodicity in the production of male and female elements. Such a want of “ time-keeping” between the sexes is called, in botanical language, dichogamy, and is one of the conditions which render self-fertilisation rarely possible. Both in plants and in animals, the male function has in the majority of cases the precedence. ‘Thus “ protandrous dichogamy ” (stamens taking the lead) is very much commoner than ‘‘protogynous dichogamy,” where the carpels are first of allmatured. This agrees with the curious cases of Angtostomum and Cymothoide already mentioned, where the organ was first male and then female, and indeed with at least most cases among closely hermaphrodite animals. Where the male organs are situ- ated in one part of the body, and the females in another, there is less reason against the production of sperms going on at the same time as the production of ova. The very physiological condi- tions which first determined the position of the ovaries here and the testes there, may remain to render it possible for the two opposing functions to go on at the same time. The common snail (/e/ix) is not only easily dissected, but in the complexity of its arrangements is full of interest. Here, not only are ova and sperms produced within the compass of one small organ, but each little corner of the organ shows female cells forming on the walls and male cells in the centre. It has been justly suggested by Platner that the outer cells are the better nourished; they therefore naturally become developed into anabolic ova. § 7. Self-Fertilisation.—We have noted above, that though male and female organs be present in the same organism, they tend to become mature at different times, and that the more the closer the seats of formation of the two kinds of elements. 74 THE EVOLUTION OF SEX. It is equally necessary to emphasise, that though both male and female elements may be produced in the same plant or animal, it is probably exceptional for the ovule to be penetrated by a pollen cell from the same flower, and it is certainly rare for an animal to fertilise its own ova. It is believed by breeders of higher animals that “ close- breeding” beyond a certain point is dangerous to the welfare of the breed. ‘The offspring tend to be abnormal or unhealthy. In view of this, the rarity of self-fertilisation among herma- phrodites has been explained in terms of the disadvantage of the process. In reality, however, this is putting the cart before the horse. In hermaphrodites, we take it that the two kinds of sexual elements mature and are liberated at different times, not because of any reaction of the disadvantageousness of self fertilisation on the health of the species, but simply because the simultaneous co-existence of opposite physiological processes 1s in varying degrees prohibited. More technically, dichogamy is not a subsequent result of the disadvantage of self-fertilisation, but cross-fertilisation is the subsequent result of increasing dichogamy. Self-fertilisation does, however, occur as an exception among animals,—thus in all probability in the exceptional fish Serranus ; certainly in many parasitic flukes or trematodes ; ‘““commonly, if not universally,” in tape-worms or cestodes ; also in the curious thread-worm Axgiostomum, and probably in ctenophores, and in some other cases. In regard to some cases, é.g., among hermaphrodite bivalves (where the sperms are usually wafted in with the water), it is impossible as yet to say whether self-impregnation does or does not occur. Some curious, but not very reliable, observations are on record in regard to self-impregnation in casually hermaphrodite insects. Arguing from the bad effects of close breeding among higher animals, Darwin and others have called attention to the numerous contrivances among plants which are said to render self-fertilisation impossible. It must again be said, that this survival of a very old way of explaining facts—in terms of their final advantage—is not really a causal explanation at all. It has been pointed out, that in some cases the pollen of a given flower is quite inoperative on the ovule of the same flower, or has the result of producing weakly offspring. ‘Then there are a great variety of mechanical devices, as the result of which it is more or less physically impossible for the pollen of the HERMAPHRODITISM. 75 stamens to reach the stigmas of the flower, or even to be dusted upon them by the unconscious agency of the intruding insects. Moreover, as among animals, so among plants, it is common for the male organs to become mature before the carpels are ready, or, in rarer cases, for the reverse to occur. There is no doubt that cross-fertilisation very generally occurs, and it is physiologically probable that this is a con- siderable advantage, though less among plants (which are so very “female,” z.c., vegetative) than among animals. But there is an Increasing impression that both the occurrence of cross- fertilisation, and the necessity of it among higher plants, hav been exaggerated by the extreme Darwinian school. One of the most thoughtful and observant of American botanists, Mr T. Meehan, has raised a vigorous protest against the prevalent view. In the Yucca, or Adam’s needle, which is regarded as cross-fertilised by insects, he showed by experiment that there was in each flower “no abhorrence of its own pollen.” “Even when fertilised at all by insects, I am sure the fertilisation is from the pollen of the same flower.” Then as to mechanical contrivances, he says, ‘‘ we are told that iris, campanula, dandelion, ox-eye daisy, the garden pea, lobelia, clover, and many others, are so arranged that they cannot fertilise themselves without insect aid. I have enclosed flowers of all these named in fine gauze bags, and they produced seeds just as well as those exposed.” We cannot here enter into a full statement of Meehan’s careful observations, but his three main propositions well deserve statement and due consideration :— 1. Cross-fertilisation by insect agency does not exist nearly to the extent claimed for it. _ 2. Where it does exist, there is no evidence that it is of any material benefit to the race, but to the contrary. 3. Difficulties in self-fertilisation result from physiological disturbances, that have no relation to the general welfare of plants as species. § 8. Complemental Males—When Mr Darwin was inves- tigating barnacles and acorn shells, in preparation for his monograph on the group, he discovered the remarkable fact that some of the hermaphrodite individuals carried minute males concealed under their shells. These he regarded as advantageous accessory forms, ensuring cross-fertilisation in the hermaphrodites which harbour them. The great majority of 76 THE EVOLUTION OF SEX. the cirripedes are hermaphrodite; but among the barnacles proper,—the stalked forms, which are nearer the ancestral type, —separate sexes sometimes occur. On the females of a few of these, pigmy males, like those found upon hermaphrodites, also occur. These pigmy males, whether on females or herma- phrodites, are not only dwarfish, but are very often degenerate, sometimes wanting (according to Darwin) both alimentary canal and thoracic legs. Some of them, in fact, are little more than parasitic testes. (1.) The original state of affairs in this = was probably the ordinary crustacean condition of separate sexes. (2.) The males, as in some of the ‘“* water-fleas” or copepods, tended to be smaller,—smaller indeed to a Myzostomata :—A hermaphrodite (z) and a pigmy male (2).—From Nansen. vanishing point,—while the females became more and more sluggish, and settled down. (3.) In the genera Alcippe and Cryptophialus, in the species Lbla cummingiti and Scalpellum ornatum, we find true females, with attached HERMAPHRODITISM. 7k) pigmy males, often several, leading a shabby existence as parasites. (4.) In other species of Scalpellum and /bla the same pigmy males occur, but attached, as we have noted, to hermaphrodites, which in these forms have replaced the true females. (5.) Lastly, in many genera, like Pollicipes, only hermaphrodites occur. What Darwin did for the cirripedes, Graff has done for another very curious set of animals, the Myzostomata. These are degenerate chzetopods or bristle-footed worms, which occur as outside parasites on sea-lilies (crinoids), on the arms of which they make curious galls. . The majority are hermaphrodite, but again some species have the sexes separate, and again in a few cases complemental males have been demonstrated. If the hermaphrodite condition was here primitive, it persists in the majority of cases; thus, AZyzostoma glabrum is hermaphrodite, with a minute com- plemental male ; AZ. cystzcolum has the sexes distinct, but the female is just emerging from (or approaching) hermaphroditism, for it includes rudi- mentary testes; in JZ. ¢enuzspinum, tnflator, murrayz, there are separate sexes, with the females predominating in size. One conclusion, at least, is vividly suggested by these curious facts, the tendency of the male form to become reduced to a vanishing point. § 9. Conditions of Hermaphroditism.—In looking back over the cases where normal hermaphroditism occurs, a few general conclusions are readily drawn. Thus Claus points out that hermaphroditism finds most abundant expression in sluggish and fixed animals. Flat-worms, leeches, earthworms, tardi- grades, land snails, &c., well illustrate the first of these; and among sponges, sea-anemones, corals, polyzoa, bivalves, &c., we find frequent illustration of the association of fixedness and hermaphroditism. Most of the tunicates are also fixed, and all are hermaphrodite. Claus notes further, how in flukes and tapeworms hermaphroditism is associated with isolated habit of life. The isolation, however, is only sometimes true, for flukes may occur near one another in great numbers ; and as many as ninety tapeworms (othriocephalus) have been known to occur at one time in a single host. Simon has gone further, in insisting on the real connection between quiescent and parasitic habit and the hermaphrodite condition. In flukes and tapeworms, leeches, Myzostomata, and some cirripedes, we find the association of hermaphroditism with a more or less intimate parasitic habit. It will be remem- bered, too, that the hagfish, in which hermaphroditism is common, is also to a large extent a parasite. But what Simon points out is, that organisms on which great demands are made, especially in the way of muscular exertion, cannot afford to be hermaphrodite ; while a plethora of nutrition, as in parasitism, tends to make the persistence of the double state possible. He gives numerous illustrations of this very reasonable contention. 78 THE EVOLUTION OF SEX. For it seems plausible that, with more available material for internal differentiation, such should actually occur. But it is possible to venture still further. A sluggish habit is usually associated with a large surplus of nutritive material, and at the same time very frequently with an accumulation of waste products. Parasitism means not only abundant, but rich and stimulating nutrition. Conditions which combine these two factors will tend to secure the persist- ence of primitive hermaphroditism, or even to develop it from a previously attained unisexual state. It must be noted, however, that exceptions occur, which it is at present difficult to explain. The ctenophores are all hermaphrodite, yet very active. So too are not a few tunicates; while the brachiopods are extremely passive, but not specially characterised by hermaphroditism. § 10. Origin of Hermaphroditism.—TVhere can be very little doubt that hermaphroditism was the primitive state among multicellular animals, at least after the differentiation of sex- elements had been accomplished. In alternating rhythms, eggs and sperms were produced. ‘The organism was alternately male and female. Ofthis primitive hermaphroditism, there is probably more or less of a recapitulation in the life-history of all animals. Gegenbaur states the common opinion in the following cautious and terse words :—“ The hermaphrodite stage is the lower, and the condition of distinct sexes has been derived from it.” Unisexual ‘‘ differentiation, by the reduction of one kind of sexual apparatus, takes place at very different stages in the development of the organism, and often when the sexual organs have attained a very high degree of differentiation.” The first structural stage in the separation would probably be the restric- tion of areas, in which the formation of two kinds of cells still went on at different times in one organism. In different in- dividuals the opposite tendencies we have already spoken of more and more predominated, till unisexuality evolved out of hermaphroditism. We may in brief suggest as the three probable grades in the history :— (az) The liberation of unindividuated sex-elements ; (4) the formation of two diverse kinds of sex-elements, incipiently male or female, at the same time, or at different periods, according to nutritive and other conditions ; (c) the unisexual outcome, where the production of one set of elements has pre- ponderated over that of the other. As at present existing, hermaphroditism may be interpreted as a persistence of the primitive state, or as a reversion to it. Individual cases must be judged by themselves, and the history HERMAPHRODITISM. 79 of each must be taken into account. But where the hermaph- roditism is manifestly exceptional, there can be seldom any question in regarding it as a reversion. ‘The reversion would generally occur on the female side, for on @ przorz physiological grounds, it is, as Simon remarks, more readily intelligible that a female should produce sperms, than that a male should produce ova. In this connection it is interesting to notice how Brock, in regard to the development of the reproduc- tive organs of snails, maintains that they are laid down and developed on the female type, and only become secondarily hermaphrodite. Purely female forms still occasionally occur, which he interprets as exaggerations of the side normally preponderant. So in hermaphrodite bony fishes, the same _author has shown that the preponderance is distinctly female. Hermaphroditism is associated in some cases (eg., Polyzoa) with the occurrence of parthenogenesis in allied forms; and it may be noted, as will become clearer afterwards, that for a female to become hermaphrodite is a sort of step towards parthenogenesis. It means that certain cells of the reproduc- tive organs are able to divide of themselves,_—to form, however, not an embryo, but a bundle of sperms. The general conclusion then is, that hermaphroditism is the primitive condition, and that the cases now existing either indicate persistence or reversion. 80 THE EVOLUTION OF SEX. SUMMARY. 1. Hermaphroditism is the union of the two sexual functions in one organism. This occurs, however, in varying degrees. 2. Embryonic hermaphroditism is probably a general fact with even unisexual animals. It is certain in some cases. 3. Casual or abnormal hermaphroditism is not infrequent. 4. Partial hermaphroditism (not involving the essential organs) is exceedingly common. 5. Normal adult hermaphroditism ; review of its occurrence. 6. True hermaphroditism occurs in many degrees of intimacy. 7. Self-fertilisation is a rare exception among animals; commoner in plants. 8. ‘*Complemental males’? — pigmies atiached to hermaphrodites— occur in two groups. 9. The conditions of hermaphroditism, in part, involve a surplus of material. 10. Hermaphroditism is primitive ; the unisexual state is a subsequent d fferentiation. The present cases of normal hermaphroditism imply either persistence or reversion. LICE RALURNE: See already cited works of GEGENBAUR, HENSEN, HERTWIG, HATCHETT JACKSON and ROLLESTON, passim. BouRNE.-—On Certain Abnormalities in the Common Frog. 1. The Occurrence of an Ovotestis. Quart. J. Micr. Sci., XXIV. Brock.—Morph. Jahrb., IV. Beitrage zur Anatomie und Histologie der Geschlechtsorgane der Knochenfische. : GILES.—Quart. Journ. Micr. Sci. 1888. LAULANIE, F.—Comptes Rendus, CI. (1885), pp. 393-5. MARSHALL, A. MILNEs.—On Certain Abnormal Conditions of the Reproductive Organs in the Frog. Journ. Anat. Physiol., XVIII., pp. 121-44. ..EEHAN, T.—On Self-Fertilisation and Cross-Fertilisation in Flowers. Penn. Monthly, VII. (1876), pp. 834-43. PFLUGER, E.—Archiv. ges. Physiol., XXIX. Stmpson, J. Y.—Todd’s Cyclopzdia of Anatomy and Physiology. Art. Hermaphroditism, pp. 684-738 (1836-9). SPENGEL.—Arb. Wiirzburg, III., 1876. Ueber d. Urogenital System der Amphibien. Zwitterbildung bei Amphibien. Biol. Centrlbl., IV., 8, cf. 9. SuTToON, J. B.—Hypertrophy and its Value in Evolution. Proc. Zool. Soc., London, 1888, pp. 432. General Pathology. London, 1886. CHAPTER (VII: THE ULTIMATE SEX-ELEMENTS (General and Historical). In our analysis of sex-characters we have followed the general course of biological history. We first passed from the form and habit of a male or female organism to the structure and functions of the sexual ovgans. In discussing hermaphroditism, we had occasion to refer to a third step of biological analysis, that which involves an investigation of the properties of the / Mies LED Cy //; | WV Mammalian ovum, showing nucleolus (a), nucleus (4), yolk (c), external porous zone or zona pellucida (d), and follicular cells (e¢):—From Hertwig, after Waldeyer. tissues. Now it is necessary to penetrate deeper, namely, to the sex-cells After these have been considered, not only in them- selves, but finally and fundamentally in terms of the changes in the protoplasm that make them what they are, then we shall be in a better position to re-ascend to some of the problems of reproduction. F 82 THE EVOLUTION OF SEX. $1. Zhe Ovum Theory.—lt is now a commonplace of observation and established fact, that all organisms, reproduced in the ordinary way, start in life as single cells. We see insects laying their ova upon plants, or fishes shedding them in the water, and may watch how these cells, provided they be fertilised, give rise eventually to the adult organisms. Con- veniently in the ordinary frog-spawn from the ditch, we can read, what was for so long a riddle, how development proceeds by successive cell-divisions and by arrangement of the multiple results. Readily seen in many instances, it is true of all cases of ordinary sexual reproduction, that the organism starts from the union of two sex-cells, or that it is with the division of a fertilised ovum that development begins. This profound fact, technically known as the “ovum theory,” has been not unjustly called by Agassiz “the greatest discovery in the natural sciences of modern times.” We shall the better realise the magnitude of the difference which its recognition has introduced into biology, if we briefly review the history. § 2. The History of Embryology, Evolution and Epigenesis. —The development of the chick, so much studied in embryo- logical laboratories to-day, was the subject of inquiry two thou- sand yearsago in Greece. Some of the conspicuous marvels of reproduction and development were persistently but fruitlessly speculated about throughout centuries. It was only during the scientific renaissance of the seventeenth century that the inquiry became more keen and sanguine, and began to rely to some extent at least on genuine observation. (z) Harvey (1651), with the aid of magnifying glasses (~erspecille), demonstrated in the fowl’s egg the connection between the ccatricula of the yolk and the rudiments of the chick, and also observed some of the stages of uterine life in mammals. More important, however, were his _far- sighted general conclusions,—(1.) That every animal was pro- duced from an ovum (ovum esse primordium commune omnibus animalibus); and (2.) That the organs arose by new formation (epigenesis), not from the mere expansion of some invisible pre- formation. In this generalisation, without however any abandon- ment of the hypothesis of spontaneous generation of germs, he strove, as he said, to follow his master Aristotle, and was in so doing as far ahead of his contemporaries as a strong genius usually is. Before Harvey, the observational method had ce THE ULTIMATE SEX-ELEMENTS. 83 indeed begun. Thus, as Allen Thomson notes, Volcher Coiter of Groningen (1573), along with Aldrovandus of Bologna, had watched the incubated egg in its marvellous progress from day to day. Fabricius ab Aquapendente (1621) had also studied the changes in the incubated egg, and the stages of the mammalian foetus. In keenness of vision, Harvey was far ahead of either of these. (6) Malpighi (1672), using a microscope with phenomenal skill, traced the embryo back into the recesses of the cicatricula or rudiment, but again missed a magnificent discovery, and supposed the rudiments to have pre-existed in the egg. In 1677, Leeuwenhock was led by Hamm to the discovery of the spermatozoa; and this was followed up, though not to much profit, by Vallisneri and others. Steno, too, in 1664, had given the ovary its present designation ; and De Graaf had interpreted the vesicles of this organ, which now bear his name, as for the most part equivalent to the ova which he had dis- covered in the oviduct. Needham (1667), Swammerdam (1685), and J. van Herne, also contributed items of information not then appreciated in their real relations. (c) Lhe Theory of Preformation—Ovists and Animalcutists. —In the early part of the eighteenth century, the embryological observations of investigators, like Boerhaave, were summed up in the conception that development was merely an expansion or unfolding of a pre-existent or preformed rudiment within the egg. Harvey had indeed striven for an opposite conclusion, but his view was negatived, as we have seen, by Malpighi’s failure to trace the embryo beyond the rudiments of the cicatricula. The notion of a preformed rudiment, thus suggested by Boerhaave, Malpighi, and others, rapidly became the prevalent - theory. In so far as it emphasises one side of the facts, it is bound in modified form so to remain. Leibnitz, Malebranche, and others found it to fit in better with their cosmic concep- tions than the older view of Aristotle had done, and welcomed it accordingly. The positions occupied by the physiologist Haller well illustrate the alterations of opinion. As Allen Thomson points out in his article ‘“‘EmMpryonocy,” in the Zucyclopedia Britannica, ‘‘ Haller was originally educated as a believer in the doctrine of ‘preformation’ by his teacher Boerhaave, but was soon led to abandon that view in favour of ‘ epigenesis’ or Se w 84 THE EVOLUTION OF SEX. new formation. But some years iater, and after having been engaged in observing the phenomena of development in the incubated egg, he again changed his views, and during the remainder of his life was a keen opponent of the system of epigenesis, and a defender and exponent of the theory of ‘evolution,’ as it was then named.” The preformation theory found more and more definite expression in the works of Bonnet, Buffon, and others. It is now necessary to sum up its main propositions. The germ, whether egg-cell or seed, was believed to bea miniature model of the adult. ‘‘ Preformed” in all trans parency lay within the egg the future organs, only requiring to be unfolded. Bonnet affirmed, that before fertilisation there lay within the fowl’s ovum an excessively minute but complete chick. They compared the germ to a complex bud, which hides within its hull the floral organs of the future. Harvey had said, ‘‘the first concrement of the future body grows, gradually divides, and is distinguished into parts; not all at once, but some produced after the others, each emerging in its order.” Very different was Haller’s first and last utterance, “There is no becoming; no part of the body is made from another, all are created at once.” This was obviously a short and easy method with embryology, if the organism was literally preformed in the germ, and its development simply a growth and an unfolding. But this was not all. The germ was more than a marvellous bud-like miniature of the adult, it necessarily included in its turn the next generation, and this the next—in short all future generations. Germ within germ, in ever smaller miniature, after the fashion of an infinite juggler’s box, was the corollary logically appended to this theory of preformation and unfold- ing,—of evolution, as it was then called, in a very different but more literal sense from that in which we now use the word. A side controversy of the time arose between two schools, who called each other “ovists” and “animalculists.” The former maintained that the female germ element was the more important, and only required to be as it were awakened by the male element to begin the process of unfolding. ‘The animal- culists, on the other hand, asserted the claims of the sperm to be the bearer of the miniature nest of organism within organism, and supposed that it only required to be fed by the ovum to enlarge and unfold the first of the models which it concealed. THE ULTIMATE SEX-ELEMENTS. 85 (2) Wolff’s Reassertion of Epigenesis—The above ingenious construction was rudely shaken down, however, in 1759, when Caspar Friedrich Wolff showed, in his doctorial dissertation, the illegitimacy of the suppositions which lay at the root of the SE Soom 2 Y Pesan : “ ) Dy ro The first stages of development in a number of animals.