_Fertility of Domesticated Varieties._--Some writers have sought to explain the contrast between domesticated varieties and natural species in respect of fertility when crossed, by the consideration that it is only those natural species which have proved themselves so far flexible as to continue fertile under changed conditions of life that can have ever allowed themselves to become domesticated. But although this condition may well serve to explain the unimpaired fertility under domestication of such species as for this very reason have ever become domesticated, I fail to see how it explains the further and altogether different fact, that this fertility continues unimpaired between all the newly differentiated morphological types which have been derived from the original specific type. It is one thing that this type should continue fertile after domestication: it is quite another thing that fertility should continue as between all its modified descendants, even although the amount of modification may extend much further than that which usually obtains between different natural species. _Testing for Cross-infertility_ among varieties growing on the same area is a much more crucial line of verification than testing for unimpaired fertility between allied species which occupy different areas, because while in the former case we are dealing with "incipient species" with a view to ascertaining whether the divergence which they have already undergone is accompanied by physiological isolation, in the latter case we can never be sure that two allied species, which are now widely disconnected geographically, have always been so disconnected. They may both have originated on the same area; or one may have diverged from the other before it migrated from that area; or even if, when it migrated, it was unchanged, and if in its new home it afterwards split into two species by physiological selection, the newer species would probably prove infertile, not only with its parent type, but also with its grand-parent in any other part of the world.