As we should expect, the passing from the sexless mode of reproduction to the definite male and female types is not sharply defined or abrupt. Even among many unicellular organisms the process becomes more elaborate with distinct specialisation of reproductive elements. In some cases conjugation is observed, when two individuals coalesce, and each cell and each nucleus divides into two, and each half unites with the half of the other to form a new cell. This is asexual, since the uniting cells are exactly similar, but the effect would seem to be the strengthening of the cells by, as it were, introducing new blood. In somewhat more complex cases these cells do not part company when they divide, but remain attached to one another, and form a kind of commonwealth. Here one can see at once that some cells in a little group will be less advantageously placed for the absorption of nourishment than others. By degrees this differentiation of function brings about differentiation of form, and cells become modified, in some cases, to a surprising extent, to serve special purposes. The next advance is when the uniting cells become somewhat different in themselves. In the early stages this difference appears as one of size; a small weakly cell, though sometimes propagating by union with a similar cell, in other cases seeks out a larger and more developed cell, and by uniting with it in mutual nourishment becomes strong. This may be seen among the protozoa where we can trace the distinct beginnings of the male and female elements. A very instructive example is furnished by the case of volvox, a multicellular vegative organism of very curious habits. The cells at first are all alike; they are united by protoplasmic bridges and form a colony. In favourable environmental conditions of abundant nutrition this state of affairs continues, and the colony increases only by multiplication and without fertilisation. But when the supply of food is exhausted, or by any cause is checked, sexual reproduction is resorted to, and this in a way that illustrates most instructively the differentiation of the female and male cells. Some of the cells are seen accumulating nourishment at the expense of the others and grow larger, and if this continues, cells which must be regarded as ova, or female cells, result; while other cells, less advantageously placed with more competitors struggling to obtain food, grow smaller and gradually change their character, becoming, in fact, males. In some cases distinct colonies may in this way arise, some composed entirely of the large well-nourished cells, and others of small hungry cells, and may be recognised as completely female or male colonies.[13]
We are now in a position to gain a clue to the difficult problem of the origin of the sexes. It would be easy as well as instructive to accumulate examples.[14] I am tempted to linger over the life-histories of these early organisms that are so full of suggestion; but the case I have selected—the volvox—really answers the question. Sex here is dependent on, and would seem to have arisen through, differences in environmental conditions. We find the well-nourished, larger, and usually more quiescent cell is the female, the hungrier and more mobile cell the male; the one concerned with storing energy, the other with consuming it, the one building up, the other breaking down; or expressed in biological formula, the female cell is predominantly anabolic, that of the male predominantly katabolic. Thus we find that the male, through a want of nutrition, was carried developmentally away from the well-fed female cell, which it was bound to seek and unite with to continue life. This relation between the food supply and the sexes is found persisting in higher forms, and, in this connection, the well-known experiments of Young on tadpoles and of Siebald on wasps may be cited. By increasing the nutrition of tadpoles the percentage of females was raised from the normal of about fifty per cent. to ninety, while similarly among wasps the number of females was found to depend on warmth and food supply, and to decrease as these diminished. Mention also may be made of the plant-lice, or aphides, which infest our rose-bushes and other plants, which, during the summer months, when conditions are favourable, produce generation after generation of females, but on the advent of autumn, with its cold and scarcity of food, males appear and sexual reproduction takes place. Similarly brine-shrimps when living under favourable conditions produce females, but when the environment is less favourable males as well are found. Another significant fact is the simple and well-known one that within the first eight days of larval life the additions of food will determine the striking and functional differences between the workers and queen-bee.[15] Among the higher animals the difficulties of proving the influence of environment upon sex are, of course, much greater. There are, however, many facts which point to a persistence of this fundamental differentiation. Among these it is sufficient to mention the experiments of stock-breeders, which show that good conditions tend to produce females; and the testimony of furriers that rich regions yield more furs from females, and poor regions more from males. Even when we reach the human species facts are not wanting to suggest a similar condition. It is usual in times of war and famine for more boys to be born; also more boys are born in the country than in cities, possibly because the city diet is richer, especially in meat. Similarly among poor families the percentage of boys is higher than in well-to-do families. And although such evidence is not conclusive and must be accepted with great caution, it seems safe to say that the facts—of which I have given a few only of the most common—are sufficient to suggest that the relation among the lower forms of life persists up to the human species, and that the female is the result of surplus nutrition and the male of scarcity.
This is sufficient for our present purpose; all other questions and theories brought forward regarding the determination and conditions of the sexes are outside our purpose. Those who will survey the evidence in detail will find ample confirmation of the point of view I wish to make clear. (1) All species are invented and tolerated by Nature for parenthood and its service; (2) the demands laid upon the female by the part required from her are heavier than those needed for the part fulfilled by the male. The female it is who is mainly responsible to the race. And for this reason the progress of the world of life has always rested upon and been determined by the female half of life. What I wish to establish now is that the male developed after and, as it were, from the female. The female led, and the male followed her in the evolution of life.
FOOTNOTES:
[8] Haeckel, Generelle Morphologie der Organismen, Vol. II. p. 16.
[9] Thomson, J. Arthur, Heredity, p. 29.
[10] Thomson, J. Arthur, Heredity, p. 33.
[11] Ward, Pure Sociology, p. 307.
[12] See Ward, op. cit., pp. 304-314, from whose chapter on this subject I have taken these facts.