_Cross-infertility in Domesticated Cattle._--Mr. J. W. Crompton, who has had a large experience as a professional cattle-breeder, writes to me (March 2, 1887)-- "That form of barrenness, very common in some districts, which makes heifers become what are called 'bullers'--that is, irregularly in 'season,' wild, and failing to conceive--is certainly produced by excess of iron in their drinking-water, and I suspect also by a deficiency of potash in the soil." He also informs me that pure white beasts of either sex are so well known by experienced breeders to be comparatively infertile together, that they are never used for breeding purposes, so that "in some parts of the country, where a tendency to sterility had become so confirmed in the white race that they utterly died out," only the coloured breeds are now to be found. He goes on to say that if "a lot of white heifers were put to a lot of white bulls, I think you would probably get a fertile breed of pure white cattle.... I think, in short, that domestication has produced just what your theory suggests, a new variety inclined to prove sterile with its parent stock." Commenting on the origin of domesticated cattle, Professor Oscar Schmidt remarks (_Doctrine of Descent_, p. 139)-- "Rütimeyer's minute researches on domestic cattle have shown that, in Europe at least, three well-defined species of the diluvial period have contributed to their formation--_Bos primigenius_, _longifrons_, and _frontosus_. These species once lived geographically separate, but contemporaneously; and they and their specific peculiarities have perished, to rise again in our domestic races. These races breed together with unqualified fertility. In the form of skull and horns they recall one or other of the extinct species; but collectively they constitute a new main species. That from their various breeds, the three or any one of the aboriginal species would ever emerge in a state of pristine purity, would be an utterly ludicrous assertion." Now, seeing that these "aboriginal species," although living "contemporaneously," were "geographically separate," we can well understand that their divergence of type from a common ancestor did not require, as a condition to their divergence, that any cross-sterility should have arisen between them. The geographical isolation was enough to secure immunity from mutual intercrossing, and therefore, as our present theory would have expected as probable, morphological divergence occurred without any corresponding physiological divergence, as must almost certainly have been the case if such polytypic evolution had occurred on a common area. Indeed, one of the two lines of experimental verification of our theory consists in selecting cases where nearly allied species are separated by geographical barriers, and proving that, in such cases, there is no cross-sterility.