Both from Kerner's work, and still more from that of Jordan and Nägeli, I conclude that (at all events in plants) prepotency is the way in which physiological selection chiefly acts. That is to say, _sudden_ and _extreme_ variations in the way of sexual incompatibility are probably rare, as compared with some degree of prepotency. According as this degree is small or great so will be the amount of the corresponding separation. This view would show that in plants the principle of physiological selection is one of immensely widespread influence, causing (on the same areas) more or less permanent varieties much below specific rank. And when we remember on how delicate a balance of physiological conditions complete correspondency of pollen to ovules depends, we may be prepared to expect that the phenomenon of prepotency is not of uncommon occurrence. _Self-fertilization and Variability._--It occurred to Count Berg Sagnitz that, if physiological selection is a true principle in nature, vegetable species in which self-fertilization obtains ought to be more rich in constant varieties than are species in which cross-fertilization rules. For, although even in the latter case physiological isolation may occasionally arise, it cannot be of such habitual or constant occurrence as it must be in the former case. Acting on this idea, Count Berg Sagnitz applied himself to ascertain whether there is any general correlation between the habit of self-fertilization and the fact of high variability; and he says that in all the cases which he has hitherto investigated, the correlation in question is unmistakable. _Additional Hypothesis concerning Physiological Selection._--In reciprocal crosses _A_ × _B_ is often more fertile than _B_ × _A_. If hybrid _AB_ is more fertile with _A_, and hybrid _BA_ with _B_, than vice versa, there would be given a good analogy on which to found the following hypothesis. Let _A_ and _B_ be two intergenerating groups in which segregate fecundity is first beginning. Of the hybrids, _AB_ will be more fertile with _A_, and _BA_ with _B_, than vice versa. The interbreeding of _AB_ with _A_ will eventually modify sexual characters of _A_ by assimilating it to those of _AB_, while the interbreeding of _BA_ with _B_ will similarly modify sexual characters of _B_ by assimilating it to those of _BA_. Consequently, _A_ will become more and more infertile with _B_, while _B_ becomes more and more infertile with _A_. Fewer and fewer hybrids will thus be produced till mutual sterility is complete.