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Chap. XXVIII. 



CONCLUDING REMARKS. 



411 



ton* 



should acquire sterility when crossed, whilst at the same time 

 we admit that domestication eliminates the normal sterility of 

 crossed species. Why with closely allied species their reproduc- 

 tive systems should almost invariably have been modified in so 

 peculiar a manner as to be mutually incapable of acting on each 

 other — though in unequal degrees in the two sexes, as shown by 

 the difference in fertility between reciprocal crosses in the same 

 species — we do not know, but may with much probability infer 

 the cause to be as follows. Most natural species have been 

 habituated to nearly uniform conditions of life for an incom- 

 parably longer period of time than have domestic races ; and 

 we positively know that changed conditions exert an especial 

 and powerful influence on the reproductive system. Hence this 

 difference in habituation may well account for the different 

 action of the reproductive organs when domestic races and when 

 species are crossed. It is a nearly analogous fact, that most 

 domestic races may be suddenly transported from one climate 

 to another, or be placed under widely different conditions, and 

 yet retain their fertility unimpaired ; whilst a multitude of 

 species subjected to lesser changes are rendered incapable of 

 breeding. 



With the exception of fertility, domestic varieties resemble 

 species when crossed in transmitting their characters in the 

 same unequal manner to their offspring, in being subject to the 

 prepotency of one form over the other, and in their liability 

 to reversion. By repeated crosses a variety or a species may 

 be made completely to absorb another. Varieties, as we shall 

 see when we treat of their antiquity, sometimes inherit their 

 new characters almost, or even quite, as firmly as species. 

 With both, the conditions leading to variability and the 

 laws governing its nature appear to be the same. Domestic 

 varieties can be classed in groups under groups, like species 

 under genera, and these under families and orders ; and the 

 classification may be either artificial,— that is, founded on any 

 arbitrary character, — or natural. With varieties a natural clas- 

 sification is certainly founded, and with species is apparently 

 founded, on community of descent, together with the amount of 

 modification which the forms have undergone. The characters 

 by which domestic varieties differ from each other are more 



