Inbreeding 187 



inb reeding. *It must always be remembered that some hermaph- 

 roditic plants are nearly always self -fertilized without apparent 

 injury. They seem to be adapted to this process, while other 

 plants that are adapted for cross-fertilization may show injurious 

 effects if too closely inbred. 



Darwin has paid much attention to this question of inbreeding. 

 He has dealt with it extensively in his book on "Animals and 

 Plants under Domestication." His general position may be 

 summed up in the following quotations: "The evil effects from 

 close interbreeding are difficult to detect, for they accumulate 

 slowly and differ much in degree in different species, whilst the 

 good effects which almost invariably follow a cross are from the 

 first manifest. It should, however, be clearly understood that 

 the advantage of close interbreeding, as far as the retention of 

 character is concerned, is indisputable, and often outw'eighs the 

 evil of a slight loss of constitutional vigor." 



"The consequences of close interbreeding carried on for too 

 long a time are, as is generally believed, loss of size, constitu- 

 tional vigor, and fertility, sometimes accompanied by a ten- 

 dency to malformation." It is generally supposed that the 

 evil effects do not appear for several generations. "On the 

 other hand, the benefit from a cross, even when there has not 

 been any very close interbreeding, is almost invariably at once 

 conspicuous." 



Danvin says "That any evil directly follows from the closest 

 interbreeding has been denied by many persons ; but rarely, by 

 any practical breeder ; and never, as far as I know, by one who 

 has largely bred animals which propagate their kind quickly." 

 Despite the fact that crossing interferes seriously with the results 

 of the breeder who is trying to keep his strain pure, all breeders 

 practically without exception make use of crossing from time 

 to time to strengthen their stock. Darwin quotes numerous 

 cases amongst domesticated animals to show that inbreeding is 

 injurious. In several cases where the pedigree has been kept 

 with care there is distinct evidence of injury. 1 



1 For details see "Animals and Plants," Chap. XVII. 



