42 ORGANIC EVOLUTION 



tions that may arise ? There are ways in which she might 

 do so. In what ways may free intercrossing between the 

 individuals of the same species be prevented ? 



In the first place, self-fertilization is a most effective bar 

 to promiscuous intercrossing and must serve to perpetuate 

 many variations that otherwise might be swamped. This 

 would be more common among plants than among the 

 higher animals, but it could occur among the lower animals, 

 many of which are bisexual. As a rule, however, at least 

 occasional cross-fertilization is advantageous and is often 

 secured either by a reluctance on the part of the sperm to 

 fertilize the ova of the same individual, as is the case, for 

 example, in most flowering plants, or by the sperm ripening 

 either before or after the eggs of the same individual, so that 

 self-fertilization cannot occur. Yet self-fertilization does fre- 

 quently occur among both animals and plants, and when it 

 does occur it may allow certain variants to persist which 

 would be likely to be swamped by cross-fertilization. 



Interbreeding between near relatives is another thing that 

 serves to perpetuate and intensify new characters which may 

 appear. This is the same thing which among domestic ani- 

 mals and plants is called " breeding in and in " and is a most 

 effective method in artificial selection. Similar interbreeding 

 between near relatives among undomesticated forms will often 

 be helped by the fact that the individuals of any species in a 

 limited locality are likely to be closely related. An insect, 

 far example, lays its eggs on a certain food plant. When 

 these hatch it is very probable that the males and females in 

 the brood will mate together and so hand down unimpaired 

 to the offspring of the second generation the characteristics 

 they received from their parents. Among sedentary ani- 



