ME. a. J. EOMAKES ON PHTSIOLOGHCAL SELECTIOlSrI 409 



The association between specific divergence and mutual sterility 

 has therefore appeared, in a liigli degree, inexplicable ; so that, 

 in Mr. Darwin's words, "the real difficulty " presented to evolu- 

 tionists has been to explain why mutual sterility " has so generally 

 occurred with natural varieties, as soon as they have been per- 

 manently modified in a sufficient degree to take rank as species " — 

 a difficulty which he thought we were still far from solving, inas- 

 much as " we are far from precisely knowing the cause." But 

 the whole of this apparently great and inexplicable difficulty has 

 arisen on account of regarding the sterility as, in some way or 

 another, the consequence of a natural variety becoming " perma- 

 nently modified." Once let the point of view be changed, or once 

 let us see in the sterility the antecedent of the permanent modi- 

 fication, and, as it appears to me, there is an end of the matter : 

 " the real difficulty " has vanished, seeing that we are no longer 

 " far from precisely knowing the cause " of the general association 

 between sterility and divergence. But, if so, can it be said that 

 the solution of such a problem, the removal of such a difficulty, 

 or the pointing out of such a causal relation, is nothing more 

 than a re-statement of fact ? Yet this is what the objection which 

 I am considering amounts to ; for, as previously remarked, it 

 goes upon the ground of accepting my whole argument, and 

 questions only the character of that argument as an explanation. 

 It may serve to place this matter in a still clearer light if I 

 briefly indicate one important consequence of my suggested ex- 

 planation of the origin of species, and one which certainly could 

 not arise if this explanation were nothing more than a re-state- 

 ment of facts already recognized. Hitherto it has been the aim, 

 or argumentative bias, of evolutionists to disparage — and even 

 to ignore — the swamping influence of intercrossing ; for, accord- 

 ing to the supposition that sterility of species is an effect of 

 morphological divergence, it obviously follows that this swamping 

 influence of intercrossing must be held inimical to such diver- 

 gence, or to the formation of new species, According to my 

 view, on the other hand, it is just this swamping influence of 

 intercrossing that constitutes the raison d'etre of all species 

 which present any degree of sterility with allied forms. Eor, 

 according to my view, it is only this one particular variation in 

 the way of such sterility which, being in virtue of its own 

 character shielded from the swamping influence, is for this reason 

 allowed to survive: it is the one particular variation that is 



LINN. JOTJEN. — ZOOLOGr, YOL. XIX. 32 



