_Bearing of Weismannism on Physiological Selection._--If in view of other considerations I could fully accept Professor Weismann's theory of heredity, it would appear to me in no small measure to strengthen my own theory of physiological selection. For Weismann's theory supposes that all changes of specific type must have their origin in variations of a continuous germ-plasm. But _the more the origin of species is referred directly to variations arising in the sexual elements, the greater is the play given to the principles of physiological selection_[67]; while, on the other hand, the less standing-ground is furnished to the theory that cross-infertility between allied species is due to "external conditions of life," "prolonged exposure to uniform change of conditions," "structural modifications re-acting on the sexual functions"; or, in short, that "somatogenetic" changes of any kind can of themselves induce the "blastogenetic" change of cross-infertility between progeny of the same parental stock. [67] _Doctrine of Descent and Darwinism_, Eng. trans. p. 139. _Cross-infertility and Diversity of Life._--Observe that one great consequence of duly recognizing the importance of intercrossing is indefinitely to raise our estimate of the part played by the principle of cross-infertility in diversifying organic nature. For whenever in any line of descent the bar of sterility arises, there the condition is given for a new crop of departures (species of a genus); and when genera are formed by the occurrence of this bar, there natural selection and all other equilibrating causes are supplied with new material for carrying on adaptational changes in new directions. Thus, owing to cross-infertility, all these causes are enabled to work out numberless adaptations in many directions (i. e. lines of descent) simultaneously. _Cross-infertility and Stability._--The importance of sterility as a diagnostic feature is obvious if we consider that more than any other feature it serves to give _stability_ to the type; and unless a type is stable or constant, it cannot be ranked as a species. That Darwin himself attributes the highest importance to this feature as diagnostic, see _Forms of Flowers_, pp. 58, 64. _Cross-infertility and Specific Differentiation._--In their elaborate work on the many species of the genus Hieracium, Nägeli and Peter are led to the general conclusion that the best defined species are always those which display absolute sterility _inter se_; while the species which present most difficulty to the systematist are always those which most easily hybridize. Moreover, they find, as another general rule applicable to the whole genus, that there is a constant correlation between inability to hybridize and absence of intermediate varieties, and, conversely, between ability to hybridize and the presence of such varieties.