240 FERTILITY OF VARIETIES [CHAP. IX. 



remarkable. In the first place, it may be observed that the 

 amount of external difference between two species is no sure 

 guide to their degree of mutual sterility, so that similar differ- 

 ences in the case of varieties would be no sure guide. It is 

 certain that with species the cause lies exclusively in differences 

 in their sexual constitution. Now the varying conditions to 

 which domesticated animals and cultivated plants have been 

 subjected, have had so little tendency towards modifying the 

 reproductive system in a manner leading to mutual sterility, that 

 we have good grounds for admitting the directly opposite doctrine 

 of Pallas, namely, that such conditions generally eliminate this 

 tendency ; so that the domesticated descendants of species, which 

 in their natural state probably would have been in some degree 

 sterile when crossed, become perfectly fertile together. With 

 plants, so far is cultivation from giving a tendency towards 

 sterility between distinct species, that in several well-authenti- 

 cated cases already alluded to, certain plants have been affected 

 in an opposite manner, for they have become self -impotent whilst 

 still retaining the capacity of fertilising, and being fertilised by, 

 other species. If the Pallasian doctrine of the elimination of 

 sterility through long-continued domestication be admitted, and 

 it can hardly be rejected, it becomes in the highest degree im- 

 probable that similar conditions long-continued should likewise 

 induce this tendency ; though in certain cases, with species having 

 a peculiar constitution, sterility might occasionally be thus caused. 

 Thus, as I believe, we can understand why with domesticated 

 animals varieties have not been produced which are mutually 

 sterile ; and why with plants only a few such cases, immediately 

 to be given, have been observed. 



The real difficulty in our present subject is not, as it appears 

 to me, why domestic varieties have not become mutually infertile 

 when crossed, but why this has so generally occurred with natural 

 varieties, as soon as they have been permanently modified in a 

 sufficient degree to take rank as species. We are far from pre- 

 cisely knowing the cause ; nor is this surprising, seeing how pro- 

 foundly ignorant we are in regard to the normal and abnormal 

 action of the reproductive system. But we can see that species, 

 owing to their struggle for existence with numerous competitors, 

 will have been exposed during long periods of time to more uni- 

 form conditions, than have domestic varieties ; and this may well 

 make a wide difference in the result. For we know how commonly 

 wild animals and plants, when taken from their natural condi- 

 tions and subjected to captivity, are rendered sterile ; and the 

 reproductive functions of organic beings which have always lived 

 under natural conditions would probably in like manner be 

 eminently sensitive to the influence of an unnatural cross. 



