Two distinct types of horse, in many instances largely modified by interbreeding, appear to exist.
Von Buch laid stress on geographical isolation as the cause of production of varieties, the different conditions of the environment and the segregated interbreeding gradually producing local races.
It is clear, however, that through Ray's work in the 17th century the common biological application of species became fixed much in its modern form, as denoting a group of animals or plants capable of interbreeding, and although not necessarily quite identical, with marked common characters.
Such stocks, however, usually fail in time, partly from too close interbreeding, partly from the ordinary chances of mortality, and partly from the cumulative effects of strange conditions.
Mendel's object was to gain further knowledge as to lsm the result of mixing by cross-fertilization or interbreeding two strains exhibiting diverse characters or structural features.