PHYSIOLOGICAL ANIMAL GEOGRAPHY 595 
(Schimper ’03) and differ in this respect from plants of cooler, 
moister situations. Again, the difference between the tiger 
beetles which deposit their eggs in different soils is not structural 
difference in ovipositor, but a different physiological response of 
the organism. 
The activities mentioned are general and may be separated, 
into feeding, breeding, etc. Probably all are governed by the 
same general laws. In the study of all the animals of a given 
environment we are confronted with the question of what activi- 
ties are most important, just as in the study of particular species. 
2. The most important environmental relations of animals 
The strength of a chain is the strength of the weakest link. 
The activity which determines the range of conditions under which 
a species will be successful is the activity which takes place within 
narrowest limits. Failure to recognize this principle is in part 
responsible for the general chaotic state of our knowledge of 
natural history. Men have often failed to interpret the relations 
of animals to their environments because they have regarded the 
records of the occurrence of all stages of the life-history as equally 
important. They have considered the occurrence of the most 
motile stage in the life-history important, as for example, the 
occurrence of an adult butterfly. Plant ecologists would have 
met with equal success if they had studied only the environmental 
relations and distribution of wind-disseminated seeds. While 
we have indicated above that the breeding activities are most 
limited (Merriam, ’90; Allen and Verrill fide Merriam, ’90; Adams, 
’08; Shelford, ’07, 710; Reighard, ’10; Herrick, ’02; Clark, ’10), 
there are no doubt exceptions to this, and at the present stage of 
our knowledge the subject is one for investigation. Whatever 
this activity may prove to be in a given case, it is of great import- 
ance and deserves comment, both as to method of investigation 
and bearing on distribution. 
a. Method of determining the most important activities. The 
first step is field observation—the continuous watching of animals 
throughout a number of life cycles. Experimentation is almost 
