Up to this point we have considered mainly the processes of main- 

 tenance of the animal body, but there are other processes as well which 

 must be called to mind, such as growth, development, multiplication, 

 and behavior. Physiologically considered, none of these activities are 

 essentially different from the fundamental phases of metabolism and 

 all are dependent upon it; they are special forms of the transformation 

 of substances and energy within the animal. As the individual animal 

 grows and develops in its life cycle, its metabolism, form, and behavior 

 change in an orderly manner, and this transformation is in the main 

 a continuous process like the other transformations of matter and en- 

 ergy. The changes which take place during entogeny are often greater 

 than the differences which exist between very distantly related adults, 

 and these differences result in very different roles which the animal 

 often plays in the economy of nature. 



Comparable to the responses of the animal to its environment, and 

 indeed essentially of the same kind, are the responses of any part of 

 an animal to all its other parts, the entire organism, in this case, being 

 considered as a unit. The environment of an internal parasite is 

 formed by the body of its host, and in a similar sense the different 

 parts of the body are parts of the environment of the other 

 parts. The different parts of the animal body are what they are 

 on account of three conditions. The first is determined by its 

 relative position and responses as a member of a series of 

 successive generations. In this way the hereditary potential- 

 ities are determined. Ecologically considered heredity may be 

 regarded both as the response of individuals (unicellular) and 

 germs to the conditions of life, and as the mutual responses of 

 different germs to one another. The crossing and intermingling of 

 germinal elements is as truly a response as are other forms of activity. 

 Secondly, there is considerable evidence which indicates that at some 

 stage in the development of an animal any part is potentially capable 

 of developing into any other part. The character of development, 

 then, is conditioned by the character of the cell-environment — its rela!- 

 tive position, and all that implies with regard to environment. A frag- 

 ment of a regenerating animal develops differently according to its po- 

 sition, and this is a response to its relative position in the cell commu- 

 nity. Thirdly, the development of an animal is conditioned by its ex- 

 ternal environment. The external conditions influence animals by 

 changing their internal activities. The internal changes modify the 

 cell community and change development. In this manner every part 

 of the animal is influenced by the conditions of its existence. 



The processes of metabolism are continuous as long as life lasts. 

 Thus, as an animal respires there is a gaseous exchange, from the 



