Darwin had brought home with him as the result of his scientific journeys the conclusion that species of plants and animals are not fixed but are subject to variations. In order to pursue this idea he entered upon experiments in the breeding of plants and animals. Just for this reason England has become a classic land. The scientists of other countries, Germany, for example, have nothing to offer comparable with England in this respect. Moreover, most of the conclusions belong to the last century so that the establishment of the facts presented few difficulties. Darwin found that this artificial breeding produced differences in the species of plants and animals greater than occur among those which are universally recognised as belonging to different species. Therefore it was, up to a certain point, proved that species can change and furthermore there was established the possibility of a common ancestry for organisms which partake of the characteristics of different species. Darwin now examined the question whether there were not in nature causes--which without the conscious intention of the breeder--might in the course of time, by means of heredity, produce changes in the living animal analogous to those produced by scientific breeding. These causes he found in the disproportion between the enormous number of germs made by nature and the small number of beings which actually come to maturity. But as the germ struggles for its own development there is of necessity a consequent struggle for existence, which not only shows itself directly in the wear and tear of the body, but also as a struggle for space and light, as in the case of plants. And it is evident that in this fight those individuals have the best prospect of coming to maturity and reproducing themselves which possess certain qualities, perhaps insignificant, but advantageous in their fight for existence. There is a tendency towards the inheritance of these individual properties, and if they occur in several individuals of the same species towards development in the direction once taken, by virtue of the accumulated heredity, while the individuals which are not possessed of these qualities succumb more easily and little by little disappear in the struggle for existence. Thus a species naturally changes by the survival of the fittest.