Here, then, are two sharply-defined theories to explain the very general fact of there being some greater or less degree of cross-infertility between allied species. The older, and hitherto current theory, supposes the cross-infertility to be but an _accident_ of specific divergence, which, therefore, has nothing to do with _causing_ the divergence. The newer theory, on the other hand, supposes the cross-infertility to have often been a necessary _condition_ to the divergence having begun at all. Let us now consider which theory has most evidence in its favour. First of all we have to notice the very general occurrence of the fact in question. For when we include the infertility of hybrids, as well as first crosses, the occurrence of some degree of infertility between allied species is so usual that Mr. Wallace recommends experiments to ascertain whether careful observation might not prove, even of species which hybridize, "that such species, when crossed with their near allies, do always produce offspring which are more or less sterile _inter se_[19]." This seems going too far, but nevertheless it is the testimony of a highly competent naturalist to the very general occurrence of an association between the morphological differentiation of species and the fact of a physiological isolation. Now I regard it as little short of self-evident that this general association between mutual infertility and innumerable secondary, or relatively variable morphological distinctions, is due to the former having been an original and a necessary condition to the occurrence of the latter, in cases where intercrossing has not been otherwise prevented. [19] _Darwinism_, p. 169. The importance of physiological isolation, _when once fully developed_, cannot be denied, for it is evident that if such isolation could be suddenly destroyed between two allied species occupying a common area, they would sooner or later become fused into a common type--supposing, of course, no other form of isolation to be present. The necessity then for this physiological form of isolation in _maintaining_ a specific differentiation which has been already _attained_ cannot be disputed. Yet it has been regarded as "Darwinian heresy" to suggest that it can have been of any important service _during the process of attainment_, or while the specific differentiation is being advanced, and this notwithstanding that the physiological change must presumably have developed _pari passu_ with the morphological, and notwithstanding that in countless cases the former is associated with every conceivable variety of the latter.