Xos. l>22-<i2:?] MIGRATION A FACTOR IX EVOLUTION 477 



ess of humus formation reaches a certain degree of cumu- 

 lative development. Under such circumstances this later 

 stage must be preceded by antecedent processes, and the 

 restoration of the balance is long delayed. Some adjust- 

 ments take place so quickly that little can be learned of 

 the stages through which they pass. There are, however, 

 many slow processes which afford an abundance of time 

 for study; in fact some are too slow to study during a life 

 time. The processes which are moderately slow are often 

 particularly illuminating because all stages are fre- 

 quently so well preserved that comparison is a very use- 

 ful method of study ; the slowness of a process has a cer- 

 tain resolving power, as it were, recalling the influence of 

 a prism upon a beam of white light, which reveals many 

 characteristics obscure to direct vision. A study of the 

 processes of adjustment among animals is a study of an 

 important phase of the problem of maintenance. The 

 continued process of response will, if circumstances per- 

 mit, lead to a condition of relative adjustment, or bal- 

 ancing among all the factors in operation." The de- 

 termination of the dynamic status and its application to 

 cycles is seen to be a method or criterion which may be 

 used for the determination of cycles of activity, and the 

 repetition of these determinations will indicate the direc- 

 tion of movement of a process, and thus serve as a guide 

 in the determination of its rate of change. 



(c) Limiting Factors.— Animals live in a real world, 

 they are dependent upon an environment and they can 

 not be understood independently of it. They do not live, 

 as it were, in a vacuum. As Brooks ( '02, p. 485) has said : 

 "Xo physiologist who studies the waste and repair of 

 living bodies, no naturalist who knows living beings in 

 their homes, no embryologist who studies the influence of 

 external conditions upon development, can, for an in- 

 stant, admit that living beings are self-sufficient or self- 

 sustaining, or that their being is in themselves; for the 

 line we draw, for our study, between living beings and 

 the external world is not one we find in nature, but one 



