(5) The last line of argument in favor of the belief that all existing species have descended from a few simple primitive forms is found in the fact of the variation of animals through artificial selection under domestication. For generations breeders have known that by carefully selecting the type of animal or plant which they have desired, it is possible to produce approximately that type. Thus have originated all the breeds or varieties of domestic plants and animals. Now, Darwin conceived that nature also exercises a selection by weeding out those individuals that are not adapted to their environment. In other words, nature, though unconscious, selects in a negative way the stronger and the better adapted. Animals vary in nature as well as under domestication from causes not yet well understood. The variations that were favorable to survival, Darwin argued, would secure the survival, through the passing on of these variations by heredity of the better adapted types of plants and animals. The natural process of weeding out the inferior or least adapted through early death, or through failure to reproduce, Darwin called "natural selection", and likened it in its effect upon organisms to the artificial selection which breeders consciously use to secure types of plants or animals that they desire. The only great addition to Darwin's theories which has been made since he wrote is that of the Dutch botanist, Hugo de Vries, who has shown that the variations which are fruitful for the production of new species are probably great or discontinuous variations, which he terms "mutations," instead of the small fluctuating variations which Darwin thought were probably most important in the production of new species. De Vries' theory in no way affects the doctrine of descent, nor does it take away from the importance of natural selection in fixing the variations. Darwin's theory, therefore, stands in all of its essentials to-day unquestioned by men of science, and it must be assumed by the student of sociology in any attempt to explain social evolution.