Sex. In all the many-celled animals, and in most of the many-celled plants, the continuance of life from generation to generation is secured by special reproductive cells which form new lives, being sooner or later separated from the body of the parent. In this specialisation of reproductive cells lies the great difference between sexual and asexual reproduction. But in the great majority of organisms the special reproductive cells are of two kinds—there are relatively large and passive ova, and relatively small and often very active spermatozoa. These two kinds of reproductive cells are mutually dependent; in most cases they die unless they unite with one another. When this union or fertilisation takes place a new life begins. But in most organisms the ova are formed in one organ, the spermatozoa in another, while in most animals, and in all the higher forms, the ova are formed by one individual (a female) and the spermatozoa by another (the male)—there being, in other words, two distinct sexes. Moreover, the males and females differ not only in their essential characteristics of producing spermatozoa and ova respectively, but they often differ very markedly in external appearance, constitution, and habits. Further, they are instinctively attracted to one another in such a way that the union of the complementary reproductive cells is secured.
It is evident then that the fact of sexual reproduction involves many distinct biological problems: (a) the formation of specialised reproductive cells; (b) the origin of two different kinds of reproductive cells—ova and spermatozoa; (c) the differentiation of distinct organs or of two distinct types of indi- vidual for the production of these two kinds of elements; (d) the divergence between the sexes in secondary as well as in essential characteristics; and (e) the evolution of the sexual instincts which find their highest expression in the love and courtship of many of the higher animals. As the general problem of sexual reproduction is discussed in the article REPRODUCTION, and as the divergent modes of sexual reproduction are discussed in special articles, such as HERMAPHRODITISM and GENERATIONS (ALTERNATION OF), we shall confine ourselves here to the problems immediately connected with the evolution of sex.
What is the meaning of the existence of two distinct sexes? The problem may be approached in several different ways. We may, for instance, inquire whether this normal divergence of individuals into two types has any analogue among other cases of divergent variation among organisms. To some the analogy seems not far to seek. For throughout organic nature a great contrast may be read, between plant and animal, Gregarine and Infusorian, coral and jellyfish, barnacle and shrimp, coccus-insect and fly, slug and Pteropod, tortoise and lizard, reptile and bird, and even more clearly between related genera or even related species; a contrast between predominant passivity and activity, between a storing and constructive physiological habit and one which is prone to expenditure and disruption; a contrast expressible in technical language as an antithesis between a relative anabolic and a relative katabolic preponderance in the protoplasmic life of the creature. Now it may be that the difference between the sexes, between the relatively passive female and the more energetic male, is an expression of what is really a fundamental alternative in variation.
Or we may take a survey of the contrasts between the sexes, and endeavour, after making allowance for the special conditions of each case, to reach some average truth. Then we find that females tend to be larger, more sluggish, less brightly coloured and decorative, that they often have a longer life and sometimes a higher temperature; and that males, on the other hand, tend to be smaller, more energetic, more brightly coloured and decorative, and so on. Now, these characteristics, when physiologically analysed, tend to corroborate the theory that the females and males are individuals of relatively anabolic and relatively katabolic constitutions.
Again, we may approach the problem by inquiring into the physiological conditions which tend in each birth to determine whether the individual will be a male or a female. In many cases—e.g. in amphibians, birds, and mammals—the young life is for some time—usually of very short duration—potentially bisexual; it is believed that surrounding conditions, especially of nutrition, acting upon some constitutional predisposition, determine the predominance of either maleness or femaleness. It is true that we have but incomplete knowledge of this matter, but there are many facts which suggest that influences of food, temperature, &c., which favour anabolism in the parents, embryos, and young, tend to result in female offspring, while opposite factors tend to produce males. Thus by abundant and rich nutrition Jung raised the percentage of females in a brood of tadpoles from 56 to 92, and starved caterpillars tend to become male butterflies or moths. Or again, we may fix our attention on the characteristic products of the sexes—on the ova and spermatozoa. Here the difference between female and male has its fundamental and most concentrated expression. For the ova are relatively large cells, very passive, and usually rich in reserve-products, while the spermatozoa are very minute cells, usually very active, rarely with any reserve-products. In short, the ova (like the females) are relatively anabolic, and the spermatozoa (like the males) are relatively katabolic.
We should also inquire into the beginnings of sexual differentiation among the simpler forms of life. Among the unicellular Protozoa, in which sexual reproduction in the strict sense cannot occur, there are already the analogues of males and females, as we see for instance in Vorticella, when a small free-swimming individual unites with a stalked cell of normal size. Very gradual among the simple plants also are the steps of sexual differentiation. But of especial importance is the genus Volvox—a colony of loosely united cells—in many ways related to the Flagellate Infusorians. In this globular colony division of labour is at a minimum, and individuals often occur which are quite asexual, forming daughter-colonies, without specialised reproductive cells. Others have, among the other units of the colony, special reproductive cells, which are, however, parthenogenetic, able of themselves to form new colonies. Others have special reproductive cells—ova and spermatozoa—formed within the same colony, but usually maturing at different times. Then there are colonies in which only ova are formed, and others in which only spermatozoa are formed, cross fertilisation taking place as usual. And besides these various sexual conditions other combinations often occur in the same species of Volvox, in which we can indeed read, and with increasing physiological knowledge will more fully understand, almost the whole story of the evolution of sex.
Finally, it must be observed that the origin of two kinds of reproductive cells which combine in fertilisation, and the specialisation of two types of individual as the bearers of ova and spermatozoa respectively, must have been of advantage in the general evolution of organisms. For, as almost every life begins in the mingling of two distinct reproductive units produced by two distinct types of individual, there is here one of the most important sources of variation and one of the most important means of securing the average stability of the species.
But we must also recognise the psychical expression of sex—the love of mates. This also has its history. Among crustaceans and insects first, in fishes and amphibians, in reptiles too, but most markedly among birds and mammals, the males are attracted to the females, and enter into relations of helpfulness with them. The relations and attractions may be crude enough to begin with, but to mere physical fondness are added subtler attractions of sight and hearing, and these are sublimed in birds and mammals to what we call love. This love of mates broadens out; it laps the family in its folds; it diffuses itself as a saturating influence through the societies of animals and of men.
See REPRODUCTION, SEXUAL SELECTION, and The Evolution of Sex, by Prof. Geddes and the present writer (1889), with a bibliography of the subject.