432 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 distinguish 

 individuals, the result of their combined activities determining 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. 



462. 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 w^hich each individual is distinguished from others of its 

 kind. It is probably true that no two individual organisms are ever 

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

 individuality 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 maintains its 

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

 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, remain 

 the same throughout all or a large part of the individual's life, while in 

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

 continually taking place. 



463. 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 senility (Sec. 69). 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 maturity 



