"^^ THE NEW EVOLUTION 



including all the ctenophores, in the tapeworms, 

 most flukes, most turbellarians, some nematodes, the 

 arrow-worms or chastognaths, most polyzoans, pos- 

 sibly a few brachiopods, most barnacles, a few fishes, 

 and an amphibian. In a number of fishes and some 

 amphibians individuals occur casually in which both 

 sexes are equally developed. 



Both sexes are equally developed in earthworms, 

 leeches and snails, but in these creatures the indi- 

 viduals are not capable of self-fertilization. In tad- 

 poles both sexes are equally developed, but only one 

 sex continues to develop to maturity. 



In some animals, as for instance in the slime-eels 

 and some nematodes, the individuals are at first male 

 and later female, while in others, as in most tunicates, 

 the individuals are at first female and later male. 



In a number of different creatures, including some 

 nematodes, some barnacles, and the ceratioid fishes, 

 the males are wholly helpless parasites and live 

 attached to, or within, the body of the female. 



From this brief mention of some of the more impor- 

 tant methods for assuring the continuity of life from 

 one generation to the next and so on indefinitely it 

 is clear that every conceivable expedient is to be found 

 in some animal type or other. Furthermore, in every 

 animal type that form of reproduction best fitted to 

 produce the greatest number of young under the con- 

 ditions which must be met is the one adopted. 



These conditions differ very widely. In some cases 

 the greatest number of young results from the actual 

 production of relatively few which are cared for by 



