have long been objects of constant effort. Whole classes are constantly occupied in making the most of the phenomena of variability, as much for their own advantage as for the benefit of the community. If we consider the results thus obtained we can not fail to notice that the plants and animals in question have grown to differ so much from the original stock that, should we meet with them in nature, we should undoubtedly call them new species, perhaps even new genera. It follows that if man can in this way direct the phenomena of variability to his own use, the origin of species of plants and animals in nature may depend on a similar series of phenomena.
In nature, however, the selecting breeder is replaced by an automatic process—the survival of the fittest in the struggle for life. That struggle occurs in the first place between members of the same species; it is a struggle for food, light and air, for fecundation and thus for reproduction.
But such a struggle for existence, by which the fittest remain alive and gradually supplant the less fit, does not take place only between individuals of the same species; it is also waged—and perhaps more effectually—between closely allied species.
In this conception the final decision will be reached by the cooperation of very numerous circumstances. It finally leads to a sifting process, to the disappearance of many and to the selection of a few. Selection is a self-regulating phenomenon; it is nature that chooses, and the name of 'natural selection' is thus amply justified. According to Darwin and Wallace, who simultaneously formulated the principle, the origin of species is brought about by 'natural selection.' It is the counterpart of the voluntary or 'artificial selection,' to which man owes the improvement of various cultivated plants and domestic animals. The material oufr of which in both cases new races and new species are being created, is that which variability offers: the struggle for existence in nature, the breeder in his hothouses or in his kennels, shapes the material into new races, varieties and species.
We thus find ourselves compelled, whenever we wish to penetrate more deeply into nature's laboratories where new species are being fabricated, to sift most carefully the whole and complicated set of phenomena which we call variability. Only in this way may we hope to approach by means of our imagination the coming and going of different forms of organized life that have succeeded one another since the cooling of our planet, and of which only a small portion have been preserved as fossils since the Silurian epoch.
Darwin inaugurated the sifting process with wonderful sagacity. Wallace has continued the work, but has wandered away from reality (as de Vries will teach us) to a considerable extent. Darwin was well acquainted with the fact that two kinds of variability should be dis-
