We have abundant proof of this in the fact that domesticated species, which are carefully tended and fed, and so wholly withdrawn from the struggle for existence, vary enormously, and produce the most wonderful monstrosities. To what direct causes the appearance of a variety is due, is a question as yet unanswered. But Weismann's investigations have shown us that climate plays a large part in their development. Embryology teaches us, moreover, that the development of the young organism does not take place with the same uniformity in all organs, but that, on the contrary, in one period one organ, in another, another, undergoes a more rapid growth, which may be influenced by variations in food or temperature. Through such variations the development of monstrosities is explained. We know that influences of nourishment are operative in the development of the larvae of bees to workers or to queens, and we can easily conceive that other organs besides the sexual are subject to these influences. The field in which such influences may be operative is, indeed, boundless. 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 zooelogy, 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 amoeba 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 amoebae.