^^^^ THE NEW EVOLUTION "^^^ 



more rarely by young females, and very rarely by the 

 preadult or larval stages. 



As a prerequisite to sexual reproduction it is neces- 

 sary that the individuals become sexually mature. It 

 is important to remember, however, that sexual 

 maturity is quite a different thing from structural 

 maturity. It is true that in the great majority of 

 animals after sexual maturity is reached structural 

 development abruptly ceases, very shortly ceases, or 

 proceeds at a much slackened pace. In certain but- 

 terflies, indeed, structural development ends six 

 months or more before full sexual maturity is reached. 



But this is not always true. Even larval stages 

 may become sexually mature. Thus in one group of 

 the ctenophores the larva develops sexual cells which 

 mature during the summer and produce eggs which 

 develop normally into larvx which are, however, 

 smaller than the larvae produced by adults. After 

 the production of eggs has continued for some days 

 the larva loses the sexual cells, undergoes a com- 

 plicated metamorphosis, and develops into an adult, 

 when sexual cells again appear. Certain flukes also 

 may produce eggs in their larval stages. 



In nearly all animals on land the individuals are 

 divided into males and females. But this is by no 

 means always true. In many different kinds of ani- 

 mals both sexes are developed in every individual so 

 that single individuals produce fertile eggs or young. 



Both sexes equally developed and functioning at 

 the same time so that a single individual alone pro- 

 duces young are found in a number of coelenterates, 



