The Evolution of Man

by Ernst Haeckel

From these simplest forms of sexual propagation, as we can observe them to-day in the lowest Zoophytes, the Gastræads, Sponges, and Polyps, we gather most important data. In the first place, we learn that, properly speaking, nothing is required for sexual reproduction except the fusion or coalescence of two different cells—a female ovum and male sperm-cell. All other features, and all the very complex phenomena that accompany the sexual act in the higher animals, are of a subordinate and secondary character, and are later additions to this simple, primary process of copulation and fecundation. But if we bear in mind how extremely important a part this relation of the two sexes plays in the whole of organic nature, in the life of plants, of animals, and of man; how the mutual attraction of the sexes, love, is the mainspring of the most remarkable processes—in fact, one of the chief mechanical causes of the highest development of life—we cannot too greatly emphasise this tracing of love to its source, the attractive force of two erotic cells.

Throughout the whole of living nature the greatest effects proceed from this very small cause. Consider the part that the flowers, the sexual organs of the flowering plants, play in nature; or the exuberance of wonderful phenomena that sexual selection produces in animal life; or the momentous influence of love in the life of man. In every case the fusion of two cells is the sole original motive power; in every case this invisible process profoundly affects the development of the most varied structures. We may say, indeed, that no other organic process can be compared to it for a moment in comprehensiveness and intensity of action. Are not the Semitic myth of Adam and Eve, the old Greek legend of Paris and Helena, and so many other famous traditions, only the poetic expression of the vast influence that love and sexual selection have exercised over the course of history ever since the differentiation of the sexes? All the other passions that agitate the heart of man are far outstripped in their joint influence by this sense-inflaming and mind-benumbing Eros. On the one hand, we look to love with gratitude as the source of the greatest artistic achievements—the noblest creations of poetry, plastic art, and music; we see in it the chief factor in the moral advance of humanity, the foundation of family life, and therefore of social advance. On the other hand, we dread it as the devouring flame that brings destruction on so many, and has caused more misery, vice, and crime than all the other evils of human life put together. So wonderful is love and so momentous its influence on the life of the soul, or on the different functions of the medullary tube, that here more than anywhere else the “supernatural” result seems to mock any attempt at natural explanation. Yet comparative evolution leads us clearly and indubitably to the first source of love—the affinity of two different erotic cells, the sperm-cell and ovum.[^[34]]

[34] The sensual perception (probably related to smell) of the two copulating sex-cells, which causes their mutual attraction, is a little understood, but very interesting, chemical function of the cell-soul (cf. p. 58 and The Riddle of the Universe, chap. ix.)

The lowest Metazoa throw light on this very simple origin of the intricate phenomena of reproduction, and they also teach us that the earliest sexual form was hermaphrodism, and that the separation of the sexes (by division of labour) is a secondary and later phenomenon. Hermaphrodism predominates in the most varied groups of the lower animals; each sexually-mature individual, each person, contains female and male sexual cells, and is therefore able to fertilise itself and reproduce. Thus we find ova and sperm-cells in the same individual, not only in the lowest Zoophytes (Gastræads, Sponges, and many Polyps), but also in many worms (leeches and earthworms), many of the snails (the common garden and vineyard snails), all the Tunicates, and many other invertebrate animals. All man’s earlier invertebrate ancestors, from the Gastræads up to the Prochordonia, were hermaphrodites; possibly even the earliest Acrania. We have an instructive proof of this in the remarkable circumstance that many genera of fishes are still hermaphrodites, and that it is occasionally found in the higher Vertebrates of all classes (as atavism). We may conclude from this that gonochorism (separation of the sexes) was a later stage in our development. At first, male and female individuals differ only in the possession of one or other kind of gonads; in other respects they were identical, as we still find in the Amphioxus and the Cyclostomes. Afterwards, accessory organs (ducts, etc.) are associated with the primary sexual glands; and much later again sexual selection has given rise to the secondary sexual characters—those differences between the sexes which do not affect the sexual organs themselves, but other parts of the body (such as the man’s beard or the woman’s breast).

The third important fact that we learn from the lower Zoophytes relates to the earliest origin of the two kinds of sexual cells. As in the Gastræads (the lowest sponges and hydroids), in which we find the first beginnings of sexual differentiation, the whole body consists merely of the two primary germinal layers, it follows that the sexual cells also must have proceeded from the cells of these primary layers, either the inner or outer, or from both. This simple fact is extremely important, because the first trace of the ova as well as the spermatozoa is found in the middle germinal layer or mesoderm in the higher animals, especially the Vertebrates. This arrangement is a later development from the preceding (in connection with the secondary formation of the mesoderm).

If we trace the phylogeny of the sexual organs in our earliest Metazoa ancestors, as the comparative anatomy and ontogeny of the lowest Cœlenteria (Cnidaria, Platodaria) exhibit it to us, we find that the first step in advance is the localisation or concentration of the two kinds of sexual cells scattered in the epithelium into definite groups. In the Sponges and lowest Hydropolyps isolated cells are detached from the cell-strata of the two primary germinal layers, and become free sexual cells; but in the Cnidaria and Platodes we find these associated in groups which we call sexual glands (gonads). We can now for the first time speak of sexual organs in the morphological sense. The female germinative glands, which in this simplest form are merely groups of homogeneous cells, are the ovaries (Fig. 241 c). The male germinative glands, which also in their first form consist of a cluster of sperm-cells, are the testicles (Fig. 241 h). In the medusæ, which descend, both ontogenetically and phylogenetically, from the more simply organised Polyps, we find these simple sexual glands sometimes as gastric pouches, sometimes as outgrowths of the radial canals that proceed from the stomach. Particularly interesting in connection with the question of the first origin of the gonads are the lowest forms of the Platodes, the Cryptocœla that have of late been separated as a special class (Platodaria) from the Turbellaria proper (Fig. 239). In these very primitive Platodes the two pairs of sexual glands are merely two pairs of rows of differentiated cells in the entodermic wall of the primitive gut—two median ovaries (o) within, and two lateral spermaries (s) without. The mature sexual cells are ejected by the posterior outlets; the female (f) lies in front of the male (m).

Fig. 383—Embryos of Sagitta, in three earlier stages of development. (From Hertwig.) A gastrula, B cœlomula with open primitive mouth, C the same primitive mouth closed, ua primitive gut, bl primitive mouth, g progonidia (hermaphroditic primitive sexual cells), cs cœlom-pouches, pm parietal layer, vm visceral layer of same, d permanent gut (enteron), st mouth-pit (stomodæum).

In the great majority of the Bilateria or Cœlomaria it is the mesoderm from which the gonads develop. Probably the first traces of them are the two large cells that appear at the edge of the primitive mouth (right and left), as a rule during gastrulation or immediately afterwards—the important promesoblasts, or “polar cells of the mesoderm,” or “primitive cells of the middle germinal layer” (p. 194). In the real Enterocœla, in which the mesoderm appears from the first in the shape of a couple of cœlom-pouches, these are very probably the original gonads (p. 194). This is seen very clearly in the arrow-worm (Sagitta). In the gastrula of Sagitta (Fig. 383 A) we find at an early stage a couple of entodermic cells of an unusual size (g) at the base of the primitive gut (ud). These primitive sexual cells (progonidia) are symmetrically placed to the right and left of the middle plane, like the two promesoblasts of the bilateral gastrula of the Amphioxus (Fig. 38 p). A little outwards from them the two cœlom pouches (B, cs) are developed out of the primitive gut, and each progonidion divides into a male and a female sexual cell (B, g). The two male cells (at first rather the larger) lie close together within, and are the parent-cells of the testicles (prospermaria). The two female cells lie outwards from these, and are the parent-cells of the ovary (protovaria). Afterwards, when the cœlom-pouches have detached from the permanent gut (C, d) and the primitive mouth (A, bl) is closed, the female cells advance towards the mouth (C, st), and the male towards the rear. The foremost pair of ovaries are then separated by a transverse partition from the hind pair. Thus the first structures of the sexual glands of the Sagitta are a couple of hermaphroditic entodermic cells; each of these divides into a male and a female cell; and these four cells are the parent-cells of the four sexual glands. Probably the two promesoblasts of the Amphioxus-gastrula (Fig. 38) are also hermaphroditic primitive sexual cells in the same sense, inherited by this earliest vertebrate from its ancient bilateral gastræad ancestors.