178 Darwin, and after Darwin. 



habit of self-fertilization and the fact of high variability ; and 

 he says that in all the cases which he has hitherto investi- 

 gated, the correlation in question is unmistakable. 



Additional Hypothesis concerning Physiological Selection. 

 In reciprocal crosses A x B is often more fertile than 

 B x 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. 



To sustain this hypothesis it would be needful to prove 

 experimentally, (i) that hybrid forms AB are more fertile 

 with A than with B, while hybrid forms BA are more fertile 

 with B than with A [or, it may be possible that the opposite 

 relations would be found to obtain, viz. that AB would be 

 more fertile with J?, and BA with A] ; (2) that, if so, 

 effect of intercrossing AB with A is to make progeny more 

 fertile with A than with B, while effect of intercrossing BA 

 with B is to make progeny more fertile with B than with A. 

 Such experiments had best be tried with species where 

 there is already known to be a difference of fertility between 

 reciprocal crosses (e.g. Matthiola annua and M. glabra, see 

 Origin of Species, p. 244). 



