Inbreeding 187 
inbreeding. 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 outweighs 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.” 
Darwin 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 For details see ‘Animals and Plants,” Chap. XVII. 
