58 ANIMAL LIFE 



The object of this intermixing is the production of va- 

 riation. Nature demands that the offspring shall differ 

 slightly from its parents. By having the beginnings of its 

 body, the single cell from which the whole body develops, 

 composed of parts of two different individuals, this differ- 

 ence, although slight and nearly imperceptible, is insured. 

 Sex is a provision of Nature which insures variation. 



35. Sex dimorphism. As we have seen, almost every 

 species of animal is represented by two kinds of individuals, 

 males and females. In the case of many animals, espe- 



FIG. 27. Bird of paradise, male. 



cially the simpler ones, these two kinds of individuals do 

 not differ in appearance or in structure apart from the 

 organs concerned with multiplication. But with many 

 animals the sexes can be readily distinguished. The male 

 and female individuals often show marked differences, 

 especially in external structural characters. We can read- 



