All these considerations lead us to the conclusion that variability in general, but especially that variability resulting in a so-called improvement of the varieties producing it, is an accompaniment of prosperous conditions. This is a conclusion not yet reached in zoölogy, although botanists long ago recognized, in abundance of food, the most essential condition for the development of variations.
Darwinism fails to account for any need of nourishment beyond that necessary for the maintenance of the status quo of life. According to Darwin, the animal can acquire only sufficient for the repair of loss. The struggle for existence is, therefore, according to him, a struggle of self-defence, and its results could be, at the best, only the maintenance of species in their present position or, in a less favorable case, their decline, and finally their destruction. But this view is wholly false. The animal acquires not only enough to repair loss, but much more. How could the first amœba have propagated itself, if it consumed no more than it needed for mere self-maintenance, and how could evolution have taken place? We have seen that, even in the inorganic world, there is not an equality of loss and repair, but that, in osmose, the denser fluid takes up more than it gives, while the fluid that is less dense loses more than it receives, and the mutual exchange reaches the maximum possible under the existing circumstances. It is this characteristic which renders the involuntary and forced tendency of the organism to satiation independent of the amount of waste; this mechanical hunger is the spring of the insatiability of organisms, and explains to us their increase in number, the process of increasing perfection, and individual development. Without it, an eternity would not have sufficed for evolution; we should still have only a world of primitive amœbæ.
This theory of development is, then, the opposite of that ordinarily assumed. The latter asserts that increase of growth demands increase of nourishment, whereas this asserts the fact that increase of nourishment determines growth. The struggle for existence is not a struggle for the mere necessaries to maintain life, but a struggle for increase of acquisition, increase of life; it is not a struggle of defence, but an attack which only under certain circumstances becomes a defence. The rule with which we advise our friends is, "Forward! strive to better yourself!" though we may endeavor, in hypocritical spirit, to persuade to contentment those who come into competition with our interests.
The chief points, therefore, in which this theory differs from that of Darwin, are as follows:—
"The struggle for existence is really a struggle for increase of nourishment, of life; and independent of the supply of the moment, it goes on at all times, hence even in a state of abundance.
"Limitation of supply by competition leads to fixation of the species and, in the end, to its decrease and disappearance.
"Sickness, climate, and direct enemies are the destructive agencies, and must secure more propitious conditions for survivors, the stronger their effect.
"Only under conditions of prosperity can the survivors propagate largely, and perfect themselves, separating into varieties and species.
"The increase and differentiation of the organic world shows us that conditions of prosperity have been the rule, those of want the exception."
Rolph's extremely interesting chapter on Propagation traces the sexual instinct to the "mechanical hunger." The earliest example which may be adduced in support of this theory is that of the zoöspores which, by copulation, sustain life for a time under the unfavorable conditions of darkness, the thinner male representing, as does also the spermatozoön, the seeking individual suffering from want, the female representing a means of sustenance. The sex of the young organism is in like manner referred by Rolph to conditions of nourishment during development. We now come to the chapter on