GENERAL PRINCIPLES OF BREEDING. 51 



both in the vegetable and animal kingdoms, an occasional 

 intercross with a distinct individual is a law of Nature. 

 * * * In none, as I suspect, can self-fertilization go on 

 for perpetuity." This conclusion, based wholly on observed 

 facts, is just the conclusion to which the foregoing argu- 

 ment points, * * * If, then, in a self-fertilizing organ- 

 ism, and its self-fertilizing descendants, such contrasts as 

 originally existed among the physiological units are pro- 

 gressively obliterated if, consequently, there can no longer 

 ba a segregation of different physiological units in different 

 sperm-cells and germ-cells, self-fertilization will become im- 

 possible; step by step the fertility will diminish, and the 

 series will finally die out.* 



A similar view of this subject is presented by 

 Mr. Darwin in a letter published in the London 

 Agricultural Gazette of May, 1878, from which I 

 extract the following: 



I will venture to add a few remarks on the general ques- 

 tion of close interbreeding. Sexual reproduction is so essen- 

 tially the same in plants and animals that I think we may 

 fairly apply conclusions drawn from the one kingdom to the 

 other. From a long series of experiments on plants, given 

 in my book "On the Effects of Cross and Self-Fertilization," 

 the conclusion seems clear that there is no mysterious evil 

 in the mere fact of the nearest relations breeding together; 

 but that evil follows (independently of inherited, disease or 

 weakness) from the circumstances of near relations generally 

 possessing a closely similar constitution. However little we 

 may be able to explain the cau.se, the facts detailed by me 

 show that the male and female sexual elements must be dif- 

 ferentiated to a certain degree in order to unite properly 

 and to give birth to a vigorous progeny. Such differentia- 

 tion of the sexual elements follows from the parents and their 

 ancestors having lived during some generations under dif- 

 ferent conditions of life. 



"Principles of Biology," Vol. I, pp. 281, 282. 



