It is not improbable that all dogs sprang from one common source, but climate, food and cross-breeding caused variations of form which suggested particular uses, and these being either designedly or accidentally perpetuated, the various breeds of dogs arose, and became numerous in proportion to the progress of civilization.
These difficulties are further enhanced by the fact that, quite apart from any cross-breeding, the plants, when subjected to cultivation, vary so greatly in the course of two or three years from the original species from which they are directly descended that their parentage is scarcely recognizable.
These variations have arisen chiefly through cross-breeding, though not entirely so, there being a few cases upon record of the production of "sports" from tubers that have become the parents of new varieties, but authentic cases of the sporting of tubers are few and far between.
To account for the stripes on the subsequent foals, it is only necessary (now that the principles of cross-breeding are better understood) to assume that in the cross-bred chestnut mare there lay latent the characteristics of the Kattiawar or other Indian breeds, in which stripes commonly occur.
The numerous varieties of wheat now in cultivation have been obtained either by selection or by cross-breeding.
