126 HEREDITY AND SELECTION IN SOCIOLOGY 



surprising ; it is likewise entirely compreliensible tliat species 

 whicli, like those living in a state of nature, are adapted to 

 conditions of considerable stability, should, if the conditions are 

 suddenly altered, be adversely affected in their most fundamental 

 faculty. 



Domesticated species — that is to say, those which have been 

 for a long time adapted to domestication — are not exposed 

 to that uniformity and stability of conditions which obtain in 

 the case of wild species ; and this difference in the nature of en- 

 vironing conditions explains why changes of environment which 

 prove fatal to the reproductive power of the one are without 

 influence on the other. But it also explains why the reproduc- 

 tive organs of the stabler species should be more sensitive to a 

 change in the conditions of reproduction, even as they are more 

 sensitive to a change in the conditions of the vegetative life. 

 " Domestic productions, on the one 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 fertihty repeated changes 

 of conditions, might be expected to produce varieties which 

 would be little liable to have their reproductive powers injuri- 

 ously affected by the act of crossing with other varieties which 

 had originated in a hke manner."^ 



Let us consider the result of crossing two species as compared 

 with the result of crossing two varieties, independently of the 

 question of fertility. It may be said that the crossing of varieties 

 which are not too widely separated is almost always distinctly 

 beneficial, enhancing as it does the vigour and energy and repro- 

 ductive power of the descendants — ^provided, of course, that 

 neither parent is affected by a microbic disease or the like, which 

 would be transmitted to the offspring. It may even be that a 



1 The Origin of Species, p. 401. It must be remarked that aU varieties 

 are not fertile when crossed, although most of them are. Darwin cites 

 a few examples of non-fertiUty resulting from the crossing of varieties — 

 e.g., of two varieties of maize. 



