10 DABWINISM STATED BY DARWIN HIMSELF. 



we see that the injury from the close breeding of animals 

 and from the self-fertihzation of plants does not neces- 

 sarily depend on any tendency to disease or weakness of 

 constitution common to the related parents, and only in- 

 directly on their relationship, in so far as they are apt to 

 resemble each other in all respects, including their sexual 

 nature. And, secondly, that the adyantages of cross- 

 fertilization depend on the sexual elements of the parents 

 having become in some degree differentiated by the ex- 

 posure of their progenitors to different conditions, or from 

 their having intercrossed with individuals thus exposed; 

 or, lastly, from what we call in our ignorance spontaneous 

 variation. He therefore who wishes to pair closely related 

 animals ought to keep them under conditions as different 

 as possible. 



As some kinds of plants suffer much more 

 from self-fertilization than do others, so it 

 probably is with animals from too close interbreeding. 

 The effects of close interbreeding on animals, judging 

 again from plants, would be deterioration in general vigor, 

 including fertility, with no necessary loss of excellence 

 of form ; and this seems to be the usual result. 



It is a common practice with horticulturists to obtain 

 seeds from another place having a very different soil, so 

 as to avoid raising plants for a long succession of genera- 

 tions under the same conditions; but, with all the species 

 which freely intercross by the aid of insects or the wind, 

 it would be an incomparably better plan to obtain seeds 

 fif the required variety, which had been raised for some 

 generations unjier as different conditions as possible, and 

 sow them in alternate rows with seeds matured in the old 

 garden, The two stocks would then intercross, with a 

 thorough blending of their whqle grganizations, and with 



