460 ■ GENERAL CONSIDERATIONS 



and brings it into functional activity just at the time when the young 



animal requires food. 



Not only are hormones agents by which coordination between dif- 

 ferent parts of the body may be secured and integration be brought about, 

 but undoubtedly they are also active agents in differentiation. They 

 stimulate the development of the characteristics which distmguish 

 individuals, the result of their combined activities determinmg size and 

 bodily configuration. Hormones produced in the sex organs stimulate 

 the development of structures characteristic of either one sex or the 

 other- those from the testis are active in the production in various parts 

 of the body of characteristics that belong to the male; and those from 

 the ovary, in the production of characteristics that belong to the female 

 464 Individuality.— As a result of the processes associated with 

 differentiation and integration organisms acquire the differences that 

 characterize species, varieties, and races and also that which we call 

 individuality, by which each individual is distinguished from others of its 

 kind It is probably true that no two individual orgamsms are ever 

 precisely alike, though they may resemble each other very closely. 1 his 

 individuahty is maintained throughout the life of the organism even 

 though in the process of metabolism the exact composition of the body is 

 constantly changing. From this point of view an analogy has been 

 drawn between an organism and a whirlpool in a stream. At no two suc- 

 cessive moments of time is the whirlpool composed of the same material; 

 water constantly enters it and constantly leaves, and yet the appearance 

 of the whirlpool remains essentially the same. A living animal is continu- 

 ally taking in food and in that way bringing matter into its organization 

 and also continually throwing off waste, yet it constantly mamtams its 

 individual character. It has been said that the body changes once m 

 every seven years. This is a statement which is not exactly true, though 

 it suggests a truth. Some materials in the body, such as bone, remam 

 the same throughout all or a large part of the individual's he, while m 

 other structures of the body, as in the cells of the skin, replacement is 

 continually taking place. i,- u^„4. 



465 Life Cycle in Birds and Mammals.-The life cycle of the highest 

 vertebrates, including man, may be divided into three distinct periods 

 each of which is characterized by certain physical appearances and 

 functional relationships. These three periods are adolescence, maturity, 

 and senescence (Sec. 70). The period of adolescence is the time during 

 which the organism increases in size; metabolism is active and the chief 

 energy of the body is directed toward the production of an organism with 

 the adult stature, possessing the complete equipment of organs and 

 endowed with the energy necessary to carry on life most effectively 

 In most cases the reproductive function does not become active until 

 this period of the life cycle is well advanced. The period of matunty 



