Physiological Selection. 49 



in all cases, no matter how greatly the types differ, 

 they must agree in this, that when any parts of 

 these complex structures change, ever so slightly, 

 the reproductive system is almost certain to be 

 adventitiously affected, yet always thus affected in 

 the same peculiar way? 



If it be answered that the reproductive system is 

 known to be very sensitive to slight changes in the 

 external conditions of life, the answer proves too 

 much. For though this is true, yet our opponents 

 must acknowledge that the reproductive system is 

 not so sensitive, in this particular respect, as their 

 interpretation of the origin of specific infertility 

 requires. The proof of this point is overwhelming, 

 for there is the evidence from the entire range of our 

 domesticated productions, both vegetable and animal. 

 Here the amount of structural change, which has been 

 slowly accumulated by artificial selection, is often 

 much greater in amount, and incomparably more 

 rapid, than that which has been induced between 

 allied species by natural selection ; and yet there is 

 scarcely any indication of the reproductive system 

 having been affected in the particular way that our 

 opponents' theory requires. There are many in- 

 stances of its having been affected in sundry other 

 ways (chiefly, however, without any accompanying 

 morphological change) ; but among all the thousands 

 of our more or less enormously modified artificial 

 types, there is scarcely one instance of such a peculiar 

 sexual relation between the modified descendants of 

 a common type as so usually obtains between allied 

 species in nature. Yet in all other respects evolu- 

 tionists are bound to believe that the process of 



III. E 



