372 ME. G. J. EOMAITES ON PHYSIOLOaiCAL SELECTION. 



by secondary variations than when the prevention of intercrossing 

 arises from geographical barriers or from migration. !For in 

 this case, over and above the influence of independent variability, 

 there is a direct causal connection between the agency which 

 prevents intercrossing and the subsequent production of secon- 

 dary specific characters. So that, if Mr. Darwin's view of one of 

 the causes of variability be accepted, it follows that wherever 

 the primary specific distinction of sterility arises, there it is to 

 be exj)ected that an unusual crop of variations should follow by 

 way of consequence in other parts of the physiologically sepa- 

 rated progeny — variations, therefore, which, whether they happen 

 to be useful or unuseful, appear under circumstances most 

 favourable to their perpetuation as secondary specific characters. 

 I trust, then, that sufl&cient reasons have now been given to 

 justify my view that, if we take a broad survey of all the facts 

 bearing on the question, it becomes almost impossible to doubt 

 that the primary specific distinction is, as a general rule, the 

 primordial distinction. I say " as a general rule," because the 

 next point which I wish to present is that it constitutes no part 

 of my argument to deny that in some, and possibly in many, cases 

 the primary distinction may have been superinduced by the 

 secondary distinctions. Indeed, looking to the occasional 

 appearance of partial sterility between domesticated productions, 

 as well as to the universally high degree of it between genera, 

 and its universally absolute degree between families, orders, and 

 classes, I see the best of reasons to conclude that in some cases 

 the sterility between species may have been originally caused, 

 and in a much greaternmiiber of cases subsequently intensified, by 

 changes going on in other parts of the organism. Moreover, I 

 doubt not that of the agencies determining such changes, natural 

 selection is probably one of the most important. In other words, 

 I do not doubt that natural selection, by operating independently 

 on a separated portion of a species — whether the sejDaration be 

 physiological or geographical — may often help to induce sterility 

 with the parent form, by indirectly modifying the reproductive 

 system through changes which it efi'ects in other parts of the 

 organism ; and I see no reason to doubt that the same is true 

 of sexual selection, use and disuse, economy of growth, correlated 

 variability, or any other cause tending to modify the organism 

 in any of its parts, and so, in some instances, reacting indirectly 

 on the reproductive system in the way required. Here I only 



