324 ORIGIN OF SPECIES 



a tendency towards sterility between distinct species, that in 

 several well-authenticated 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 improbable 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 ap- 

 pears 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 precisely knowing the cause ; nor is this sur- 

 prising, seeing how profoundly ignorant we are in regard 

 to the normal and abnormal action of the reproductive sys- 

 tem. 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 uniform condi- 

 tions, than have domestic varieties ; and this may well make 

 a wide difference in the result. For we know how com- 

 monly wild animals and plants, when taken from their natural 

 conditions 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 un- 

 natural cross. Domesticated productions, on the other hand, 

 which, as shown by the mere fact of their domestication, were 

 not originally highly sensitive to changes in their conditions 

 of life, and which can now generally resist with undiminished 

 fertility repeated changes of conditions, might be expected 

 to produce varieties, which would be little liable to have 

 their reproductive powers injuriously affected by the act 



