17 



tends to maintain or perpetuate itself. But the stability of associations 

 is only relative, and some are much more stable than others. Naturally 

 the unstable ones are those which show succession most readily. Thus 

 if we destroy a few trees in a hardwood forest and produce a glade, a 

 large number of the characteristic animals of the dense forest will dis- 

 appear and be replaced by animals which normally frequent open 

 places ; then in a few years sprout-growth and young and suppressed 

 trees will change the conditions so much that the kind of forest ani- 

 mals which were eliminated for a time will begin to return ; and when 

 the new growth is replaced by the mature forest the animals of the 

 mature forest will return and a new equilibrium will be formed. In 

 such a forested region the glade is to be looked upon as an unstable 

 condition, which through a succession of associations will later arrive 

 at a relatively stable condition, which is able to perpetuate itself indefi- 

 nitely under existing conditions. Such an association is considered a 

 climax, or the culmination of a series of successions under existing 

 conditions. The succession of associations leading to a climax repre- 

 sents the process of adjustment to the conditions of stress, and the 

 climax represents a condition of relative equilibrium. Climax associa- 

 tions are large units, and are the resultants of certain climatic, geolog- 

 ical, physiographic, and biological conditions. 



The Dynamic Relations of the Environment 

 i. introductory 



In the preceding section we have seen that to understand animals 

 we must consider them as active living agents which are constantly 

 changing and responding to their environment. That the environment 

 of animals should also be studied as an actively changing medium has 

 not been as clearly recognized by students of plants and animals as one 

 might anticipate from its importance. Some students feel that the 

 study and understanding of the environment is not a part of zoology, 

 or at least not an essential part. Furthermore, to some of these stu- 

 dents at least, the environment seems largely chaotic, a confused un- 

 wieldy mass with no evident favorable point of attack. This view is 

 quite natural to those who have had no training and practical experi- 

 ence in recognizing the "orderly sequence" or laws of environmental 

 changes, and particularly to those who do not feel that environmental 

 relations are an essential part of their subject. By many such students 

 the environment is viewed in a manner comparable to the prevailing 

 chaotic views on weather before meteorology became a science, or on 

 taxonomy before Linnaeus, or on geology before Lyell. If one has seri- 

 ous doubts on this point, he need only turn to the standard treatises 



