322 



NATUEAL SELECTION. 



[Ch. XXXVII. 



occasionally destroyed. In the Pampas in Texas^ or in 

 Africa, where cattle have rnn wild in large herds, they have 

 acquired a nearly nniform dark-brownish red.^ A breed 

 called Niatas, seen by Darwin on the banks of the Plata, 

 has a short and broad forehead and other pecnliarities in the 

 shape of the sknll and in the projection and curvature of 

 the lower jaw. In this variety scarcely a single bone agrees 

 exactly in shape with that of the common ox. This breed, 

 which has existed for at least a century, is a good illustration 

 of the manner in which a marked variety may be formed in 

 a nearly wild state, and of the tendency of such a new race, 

 when brought into contact with other breeds, to keep distinct. 

 Such a tendency may point to the manner in which, in the 

 course of many generations, if man did not interfere, a greater 

 divergence from a common original and a more decided 

 aversion to sexual union might be superinduced. If the lapse 

 of time necessary for such transformations be very great, the 

 extinction of intermediate races will take place, by which a 

 new bar to the commingling of the nearest allied types will 

 be raised. 



In speculating on this subject, Mr. Darwin reminds us 

 that a slight change in the conditions of life is found to be 

 very generally advantageous to cultivated animals and plants, 

 although we know that great changes are injurious. So, in 

 the case of man, the invalid whose constitution will be 

 benefited by going from England to the South of Prance or 

 Madeira, may perish if transferred to Pernando Po. We 

 may easily imagine, that, although the crossing of most of 

 the varieties of cultivated plants and animals imparts strength 

 and fertility to them, yet under nature, and in the course of 

 ages, the variation may be carried so far as to modify the 

 reproductive organs, and render the formation of a fertile 

 hybrid germ impossible. f 



The refusal of many tamed animals to breed in captivity, 

 has been alluded to, and it demonstrates the susceptibility of 

 the reproductive system to be affected by a change in the 



^ Azara and others, cited Ly Darwin 

 * On Variation; p. 86. 



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