ANIMALS AND THEIR ENVIRONMENTS ADAMS. 519 



exist between very distantly related adidts, and these differences re- 

 sult in very different roles which the animal often plays in the 

 economy of nature. 



Comparable to the responses of tlie 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 ac- 

 comit of three conditions. The first is determined by its relative 

 position and responses as a member of a series of successive genera- 

 tions. In this way the hereditary potentialities are determined. 

 Ecologically considered heredity may be regarded both as the re- 

 sponse 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 consider- 

 able evidence which indicates that at so'nie 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 b}^ 

 the character of the cell-environment — its relative position, and all 

 that implies with regard to environment. A fragment of a regener- 

 ating animal develops differently according to its position, and this 

 is a response to its relative position in the cell community. Thirdly, 

 the development of an animal is conditioned by its external environ- 

 ment. 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 

 earliest stages of its existence until its maturity and death. Eggs 

 respire as surely as larvse and adults, and the chemical, physical, and 

 physiological changes within them var}^ with their growth and devel- 

 opment. Some of these changes are primarily dependent on the 

 orderly course of development during the life cycle, and are therefore* 

 irreversible processes, because no higher animal which is mature may 

 reverse its development and become young again. At different stages 

 of development different enzymes and harmones appear which mod- 

 ify the physiological conditions of gi'owth, development, and be- 

 havior. Environmental changes, persistent and uniform, or periodic 

 in character, tend to modify and alter these internal processes, and 

 are an additional source of change, which is particularly shown in 

 behavior. 



