The self-isolation of species is well illustrated when similar plants grow together, as grasses in the same field and practically on the same spot; yet year after year all the old kinds are found and no new ones such as might come from a mixture of two kinds, if they would mix. In the higher animals, where the parent forms are of different sexes, they choose mates of their own kind, and so each species remains distinct; but if in a species there are many different types, such as we find in domestic fowls, the members of the species, when free to do so, mate as readily with types quite different from their own as with individuals exactly like them, and produce offspring of intermediate types with all the essential characters of the species. In domestication individuals of distinct yet similar species are sometimes mated and produce offspring called hybrids, but these are sterile. The mule, which is a hybrid between the ass and the mare, is the most familiar animal of this kind. Hybrid, or mule, cage birds are produced by crossing the canary with several allied species. Among other domestic birds hybrids are almost unknown. =Origin of species.= Until near the close of the last century it was commonly believed that each species had been created in perfect form and that species were unchangeable; but long before that time some keen students of the natural sciences and close observers of the changes that take place in plants and animals in domestication had discovered that species were not perfectly stable and were changing slowly. Geologists established the fact that the earth, instead of being only a few thousand years old, had existed for countless centuries. Among fossil remains of creatures unlike any now known they had found also other forms which appeared to be prototypes of existing species. The idea that the forms of life now on the earth had come from earlier and somewhat different forms had occurred to several scientists more than a hundred years ago, but it was not until about 1860 that a satisfactory explanation of progressive development of forms of life was given to the world. This mode of creation is called evolution. The theory of evolution is that partly through their own inherent tendency to vary and partly through the influence of external things which affect them, all organisms change slowly; that things of the same kind, separated and living under different conditions, may in time so change that they become separate species; and that this process may be repeated indefinitely, the number of species constantly increasing and becoming more diversified and more highly developed.