GENERATION, SEX AND ONTOGENY 



221 



ents, although it may be slight and imperceptible, is in- 

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

 of the causes of variation. 



FIG. 127. Conjugation of the infusorian, Ynrti>eUa nebulifera; the smaller individual 

 at the right may be regarded as the male. (After Weismann.) 



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

 sented by two kinds of individuals, males and females. In the 

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



FIG. 128. Male bird of paradise. 



kinds of individuals may not differ in appearance or in structure 

 apart from the organs concerned with multiplication. But 

 with many animals the sexes can be readily distinguished. 



