30 THE ANATOMY OF INVERTEBRATED ANIMALS. 



functions. Each function is separated into many parts, which 

 are severally intrusted to distinct organs. To use the strik- 

 ing phrase of Milne-Edwards, in passing from low to high 

 organisms, there is a division of physiological labor. And 

 exactly the same process is observable in the development of 

 any of the higher organisms ; so that, physiologically as well 

 as morphologically, development is a progress from the gen- 

 eral to the special. 



Thus far, the physiological activities of living matter have 

 been considered in themselves, and without reference to any- 

 thing that may affect them in the world outside the living 

 body. But living matter acts on, and is powerfully aifected 

 by, the bodies which surround it; and the study of the in- 

 fluence of the " conditions of existence " thus determined 

 constitutes a most important part of physiolog3\ 



The sustentative functions, for example, can only be ex- 

 erted under certain conditions of temperature, pressure, and 

 light, in certain media, and with supplies of particular kinds 

 of nutritive matter ; the sufficiency of which supplies, again, 

 is greatly influenced by the competition of other organisms, 

 which, striving to satisfy the same needs, give rise to the 

 passive " struggle for existence." The exercise of the correl- 

 ative functions is influenced by similar conditions, and by the 

 direct conflict with other organisms, which constitutes the ac- 

 tive struggle for existence. And, finally, the generative func- 

 tions are subject to extensive modifications, dependent partlj^ 

 upon what are commonly called external conditions, and part- 

 ly upon wholly unknown agencies. 



In the lowest forms of life, the only mode of generation 

 at present known is the division of the body into two or more 

 parts, each of which then grows to the size and assumes the 

 form of its parent, and repeats the process of multiplication. 

 This method of multiplication by fission is properly called 

 generation, because the parts which are separated are sev- 

 erally competent to give rise to individual organisms of the 

 same nature as that from which they arose. 



In many of the lowest organisms the process is modified 

 so far that, instead of the parent dividing into two equal 

 parts, only a small portion of its substStnce is detached, as a 

 bud, which develops into the likeness of its parent. This 

 is generation by gemmation. Generation by fission and by 

 gemmation is not confined to the simplest forms of life, 

 however. On the contrary, both modes of multiplication are 



