The problems of the evolution and ancestry of the numerous breeds of domestic animals have not yet been finally solved. In many cases craniological methods of investigation have been used, and the results obtained have formed the basis of far-reaching conclusions. Such data are, however, open to serious criticism because the influence of environmental agencies during the life of the individual may be responsible for great variability of characterisation of the plastic animal body. The difficulty is increased by innumerable migrations of the ancestral stocks, by extensive hybridisation between distinct though closely related types, and the subsequent mixing of the various types in the modern breeds.