THE NEW EVOLUTION ^S" 



fishes is such a procedure possible after the animal 

 is fully grown. 



While in many animal types asexual reproduction is 

 carried to great lengths, it never wholly takes the 

 place of sexual reproduction. Sometimes sexual and 

 asexual generations alternate, or there may be several, 

 many, or even very many asexual generations between 

 two sexual generations, or asexual reproduction may 

 be rare, or very rare, or merely casual. 



Reproduction by females only through the develop- 

 ment of unfertilized eggs is not infrequent in the 

 animal world. In a number of different creatures 

 young are produced by females in the absence of any 

 males. This is true in all the rotifers (fig. 136, p. ^03). 

 Most rotifers, besides possessing this curious type of 

 reproduction, also are known to show — though 

 often very infrequently — the usual sexual reproduc- 

 tion. But in many kinds of rotifers males are en- 

 tirely unknown. 



Production of eggs and young by females in the 

 absence of males occurs as a regular and constant 

 feature in the life history of many different kinds of 

 phyllopod and ostracod crustaceans, in many different 

 kinds of hymenopterous and hemipterous insects, in 

 some jellyfishes, in some moths, and in a few other 

 creatures. In some of the crustaceans and in some 

 insects no males have ever been discovered. 



The production of young through the development 

 of unfertilized eggs is by no means confined to full 

 grown females. In a few kinds of flies young are 

 produced by the pupal stage, or even by the maggots. 



[^58] 



