MR. G. J. UOMAKES ON PHTSIOLOaiCAL SELECTI0T5". 401 



cause in question is the cause which I have termed physiological 

 selection. For what are the eflFects which stand to be explained ? 

 Broadly stated, these effects are simply millions and millions of 

 eases where there is a constant association between secondary 

 specific characters, whether useful or unuseful, and the primary 

 specific characters of sterility with, allied forms. Be it observed 

 that all these innumerable cases are alike in hind, however much 

 they may differ in regard to the degree of sterility. In a consi- 

 derable proportion of cases there is no sterility at all, and from 

 this zero level we encounter all degrees of it, until we reach, the 

 maximum degree, where sterility is absolute. 



Now, we have seen that these difierences are exactly what my 

 theory requires. For, 1st, in a considerable proportion of cases 

 intercrossing has been prevented by geographical barriers and 

 by migration ; in these cases, therefore, physiological selection has 

 had nothing to do with the evolution of species, which thus con- 

 tinue, as we might have expected, fertile inter se. 2nd, in many 

 other cases physioloi^ical selection must have been assisted in its 

 work of preventing intercrossing, whether by partial barriers of 

 a geographical kind, partial migrations, slight changes of climate, 

 habitat, instinct, and so forth ; in these cases, therefore, the re- 

 sulting species now continue to manifest corresponding fertility 

 between themselves, or fertility in all degrees. Hence,, if sterility 

 between allied species were always absolute, or even always con- 

 siderable, the fact would be fatal to my theory ; for this would 

 show that sterility between allied forms must have been due to 

 some cause other than the mere, but necessary, preservation of 

 one particular kind of variation, whenever it happens to arise. 

 But, as matters actually stand, we are able to explain the ab- 

 sence of sterility by the absence of physiological selection, and 

 the presence of different degrees of sterility by the presence of 

 different degrees of such selection. 



Confining, then, our attention to that large proportional num- 

 ber of cases where the association in question obtains, and disre- 

 garding the different degrees of sterility, what really stands to be 

 explained is the great and general fact of the association itself. 

 For what does this fact imply ? It implies that (the now ex- 

 plained exceptions apart), so soon as natural varieties become 

 entitled to take rank as species, they are found to be varieties 

 which, however much they may differ in other or secondary dis- 



LINN. JOURN. — ZOOLOGY, VOL. XIX. 31 



