82 Darwin, and after Darwin. 



Contrariwise, the theory of natural selection can have 

 no reason to form any such anticipation ; or rather 

 its anticipation would necessarilj'^ require to be the 

 exact opposite. For, according to this theory, the 

 cross-infertility of allied species is due, either to 

 correlation with morphological changes which are 

 being produced by the selection, or else, as Darwin 

 supposed, to " prolonged exposure to uniform con- 

 ditions of life " ; and thus, in either case, the sterility 

 variation ought to be, as a general rule at all events, 

 subsequent to the specific differentiation, and, ac- 

 cording to Darwin's view, long subsequent. Thus 

 we ought not to find that the physiological change 

 is ever, on any large or general scale, the initial 

 change ; nor ought we to find that it is, on any 

 such scale, even so much as a contemporary change : 

 there ought, in fact, to be no constant or habitual 

 association between divergence of embryo-types and 

 the concurrence of cross-infertility. 



Now, it will be my endeavour to prove that 

 there is an extraordinarily general association between 

 varietal divergence and cross-infertility, wherever 

 common areas are concerned ; and in as far as this 

 can be proved, I take it that the evidence will make 

 wholly in favour of physiological selection as the 

 piime condition to specific divergence, while at the 

 same time they will make no less wholly, and quite 

 independently, against natural selection as the unaided 

 cause of such divergence. 



I shall begin with some further quotations from 

 Nageli. 



Species may be synoical at all stages of relationship. We 

 come across varieties, scarcely distinguishable from one another, 



