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 imsured. 
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- 
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- 
