But the condition under which this increased ability to maintain life becomes possible is, that the parts shall cease to separate. While they are perpetually separating, it is clear that they cannot assume mutually subservient duties. And it is further clear that the more the tendency to separate diminishes, that is, the larger the groups that remain connected, the more minutely and perfectly can that subdivision of functions which we call organization be carried out.
Thus we see that in its most active form the ability to multiply is antagonistic to the ability to maintain individual life, not only as preventing increase of bulk, but also as preventing organization—not only as preventing homogeneous co-ordination, but as preventing heterogeneous co-ordination.
§ 10. To establish the unbroken continuity of this law of fertility, it will be needful, before tracing its results amongst the higher animals, to explain in what manner spontaneous fission is now understood, and what the cessation of it essentially means. Originally, naturalists supposed that creatures which multiply by self-division, under any of its several forms, continue so to multiply perpetually. In many cases, however, it has latterly been shown that they do not do this; and it is now becoming a received opinion that they do not, and cannot, do this, in any case. A fertilised germ appears here, as amongst higher organisms, to be the point of departure; and that constant formation of new tissue implied in the production of a great number of individuals by fission, seems gradually to exhaust the germinal capacity in the same way that the constant formation of new tissue, during the development of a single mammal, exhausts it. The phenomena classified by Steenstrup as "Alternate Generation," and since generalised by Professor Owen in his work "On Parthenogenesis," illustrate this. The egg of a Medusa (jellyfish) develops into a polypoid animal called the Strobila. This Strobila lives as the polype does, and, like it, multiplies rapidly by gemmation. After a great number of individuals has been thus produced, and when, as we must suppose, the germinal capacity is approaching exhaustion, each Strobila begins to exhibit a series of constrictions, giving it some resemblance to a rouleau of coin or a pile of saucers. These constrictions deepen; the segments gradually develop tentacula; the terminal segment finally separates itself, and swims away in the form of a young Medusa; the other segments, in succession, do the same; and from the eggs which these Medusæ produce, other like series of polypoid animals, multiplying by gemmation, originate. In the compound Polypes, in the Tunicata, in the Trematoda, and in the Aphis, we find repeated, under various modifications, the same phenomenon.
Understanding then, this lowest and most rapid mode of multiplication to consist essentially in the production of a great number of individuals from a single germ—perceiving, further, that diminished activity of this mode of multiplication consists essentially in the aggregation of the germ-product into larger masses—and seeing, lastly, that the disappearance of this mode of multiplication consists essentially in the aggregation of the germ-product into one mass—we shall be in a position to comprehend, amongst the higher animals, that new aspect of the law, under which increased individuation still involves diminished reproduction. Progressing from those lowest forms of life in which a single ovum originates countless organisms, through the successive stages in which the number of organisms so originated becomes smaller and smaller; and finally arriving at a stage in which one ovum produces but one organism; we have now, in our further ascent, to observe the modified mode in which this same necessary antagonism between the ability to multiply, and the ability to preserve individual life, is exhibited.
§ 11. Throughout both the animal and vegetable kingdoms, generation is effected "by the union of the contents of a 'sperm-cell' with those of a 'germ-cell;' the latter being that from within which the embryo is evolved, whilst the former supplies some material or influence necessary to its evolution."[[91]] Amongst the lowest vegetable organisms, as in the Desmideæ, the Diatomaceæ, and other families of the inferior Algæ, those cells do not appreciably differ; and the application to them of the terms "sperm-cell" and "germ-cell" is hypothetical. From this point upwards, however, distinctions become visible. As we advance to higher and higher types of structure, marked differences arise in the character of these cells, in the organs evolving them, and in the position of these organs, which are finally located in separate sexes. Doubtless a separation in the functions of "sperm-cell" and "germ-cell" has simultaneously arisen. That change from homogeneity of function to heterogeneity of function which essentially constitutes progress in organization may be assumed to take place here also; and, indeed, it is probable that the distinction gradually established between these cells, in origin and appearance, is merely significant of, and consequent upon, the distinction that has arisen between them in constitution and office. Let us now inquire in what this distinction consists.
If the foundation of every new organism be laid by the combination of two elements, we may reasonably suspect that these two elements are typical of some two fundamental divisions of which the new organism is to consist. As nothing in nature is without meaning and purpose, we may be sure that the universality of this binary origin, signifies the universality of a binary structure. The simplest and broadest division of which an organism is capable must be that signified. What, then, must this division be?
The proposed definition of organic life supplies an answer. If organic life be the co-ordination of actions, then an organism may be primarily divided into parts whose actions are co-ordinated, and parts which co-ordinate them—organs which are made to work in concert, and the apparatus which makes them so work—or, in other words, the assimilative, vascular, excretory, and muscular systems on the one hand, and the nervous system on the other. The justness of this classification will become further apparent, when it is remembered that by the nervous system alone is the individuality established. By it all parts are made one in purpose, instead of separate; by it the organism is rendered a conscious whole—is enabled to recognise its own extent and limits; and by it are all injuries notified, repairs directed, and the general conservation secured. The more the nervous system is developed, the more reciprocally subservient do the components of the body become—the less can they bear separating. And that which thus individuates many parts into one whole, must be considered as more broadly distinguished from the parts individuated, than any of these parts from each other. Further evidence in support of this position may be drawn from the fact, that as we ascend in the scale of animal life, that is, as the co-ordination of actions becomes greater, we find the co-ordinating or nervous system becoming more and more definitely separated from the rest; and in the vertebrate or highest type of structure we find the division above insisted on distinctly marked. The co-ordinating parts and the parts co-ordinated are placed on opposite sides of the vertebral column. With the exception of a few ganglia, the whole of the nervous masses are contained within the neural arches of the vertebræ; whilst all the viscera and limbs are contained within, or appended to, the hæmal arches—the terms neural and hæmal having, indeed, been chosen to express this fundamental division.
If, then, there be truth in the assumption that the two elements, which, by their union, give origin to a new organism, typify the two essential constituents of such new organism, we must infer that the sperm-cell and germ-cell respectively consist of co-ordinating matter and matter to be co-ordinated—neurine and nutriment. That apparent identity of sperm-cell and germ-cell seen in the lowest forms of life may thus be understood as significant to the fact that no extended co-ordination of actions exists in the generative product—each cell being a separate individual; and the dissimilarity seen in higher organic types may, conversely, be understood as expressive of, and consequent upon, the increasing degree of co-ordination exhibited.[[92]]
That the sperm-cell and germ-cell are thus contrasted in nature and function may further be suspected on considering the distinctive characteristics of the sexes. Of the two elements they respectively contribute to the formation of a fertile germ, it may be reasonably supposed that each furnishes that which it possesses in greatest abundance and can best spare. Well, in the greater size of the nervous centres in the male, as well as in the fact that during famines men succumb sooner than women, we see that in the male the co-ordinating system is relatively predominant. From the same evidence, as well as from the greater abundance of the cellular and adipose tissues in women, we may infer that the nutritive system predominates in the female.[[93]] Here, then, is additional support for the hypothesis that the sperm-cell, which is supplied by the male, contains co-ordinating matter, and the germ-cell, which is supplied by the female, contains matter to be co-ordinated.
The same inference may, again, be drawn from a general view of the maternal function. For if, as we see, it is the office of the mother to afford milk to the infant, and during a previous period to afford blood to the fœtus, it becomes probable that during a yet earlier stage it is still the function to supply nutriment, though in another form. Indeed when, ascending gradually the scale of animal life, we perceive that this supplying of milk, and before that of blood, is simply a continuation of the previous process, we may be sure that, with Nature's usual consistency, this process is essentially one from the beginning.