CHAPTER X. 

 REPRODUCTION AND EARLY DEVELOPMENT. 



103. The Adult Individual. With sufficient nutrition 

 and other suitable conditions of life, each individual 

 comes to a natural maturity. In each species there are 

 certain limits as to size and to length of life. There is, 

 to be sure, some variation in these respects, but mice or 

 horses or men do not pass beyond certain fairly well 

 defined size and age. We do not know just what the 

 cause of these limits is, but it is internal and not deter- 

 mined primarily by the outside conditions. 



We think of the adult individuals of a species when we 

 describe a species of animals. They are the normal 

 members. 



In most kinds of animals there are two kinds of adult 

 individuals: males and females. These are often quite 

 unlike in general appearance, in their special structure, 

 and in their instincts and habits. Indeed, the males and 

 females of one species are often more unlike than the 

 same sex of two different species. 



104. Reproduction : Its Nature and Necessity. We have 

 already seen (94) that some mode of reproduction is 

 necessary in order to keep a species alive, since all the 

 individuals grow old and die. Because individuals die, 

 new individuals must in some way be formed; because 

 individuals grow old and tend to lose their powers, the new 

 individuals that are produced by the old must be restored 



98 



