GENERATION, SEX AND ONTOGENY 



221 



ents, although it may be shght and imporoeptiblo, is in- 

 sured. Sex is a condition of nature whicli is one, at least, 

 of the causes of variation. 



Fig, 127. — Conjugation of the infusorian, Vorfiyella nebnUfern; the smaller imlividiial 

 at the right may be regarded as the male, (.\ftor Weismann.) 



As we have seen, almost every species of animal is repre- 

 sented by two kinds of individuals, males anrl females. In tlie 

 case of many animals, especially the simpler ones, these two 



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ilij. 1J5. — .Mali.' l)ir<i til ii;iiaili>L'. 



kinds of individuals may not differ in a):)i")earance or in structure 

 apart from tlie organs concerned witli nniltij^hcation. But 

 with many animals the sexes can be readily distinguislied. 



