MB. G. J. EOMANES OlST PHYSIOLOGICAL SELECTION. 399 



But many reasons have been given to show that most probably, 

 in a large majority of cases, the primary distinction has likewise 

 been the primordial distinction, and thus became the condition to 

 the subsequent appearance of secondary distinctions by inde- 

 pendent variability. 



Moreover, one very important reason was given to show that, 

 in all probability, the primary distinction is not only a condition 

 to the subsequent appearance of secondary distinctions, but itself 

 the cmtse of them ; for Mr. Darwin has shown that when the 

 reproductive system undergoes any variation, the consequences to 

 progeny are apt to consist in variations affecting other parts of 

 the organism. So that the prevention of intercrossing by phy- 

 siological barriers differs from such prevention by geographical 

 barriers, or by migration, in that, over and above the influence of 

 independent variability, there is a direct causal connection between 

 the agency which prevents intercrossing and the subsequent pro- 

 duction of secondary specific characters. 



Nevertheless, reasons have also been given to show that, in a 

 small minority of cases, this historical order may have been 

 reversed — the primary distinction having been superinduced by 

 the secondary, as we sometimes (though very rarely) find to have 

 been the case with our domesticated varieties, but which we 

 usually find to have been the case with genera, &c. Even, how- 

 ever, when such has been the case with natural varieties living on 

 the same area, it is the principles of physiological selection that 

 have determined the result ; for it can only have been those 

 secondary distinctions which happened to have been able to induce 

 the primary distinction that were, for this reason, allowed to sur- 

 vive. Thus in all cases where the evolution of species has not 

 been due to the prevention of intercrossing by geographical bar- 

 riers or by migration, it has probably been due to such preven- 

 tion by the principles of physiological selection. Or, otherwise 

 stated, all specific types which now display any degree of sterility 

 towards allied types, are probably so many records of the parti- 

 cular variation with which we are concerned having arisen in 

 the reproductive systems of their ancestry. For, not only has it 

 been shown, on antecedent grounds, that the occurrence of this 

 particular variation is in the highest degree probable, but it has 

 also been shown that, as a matter of actual observation, it does 

 occur in individuals, in varieties, and in species. Indeed, as 

 regards species, the argument here resolved itself into a mere 



