THE DISTRIBUTION OF ANIMALS. 241 
is that the great majority of animals now inhabiting the 
world have become adapted to their present environ- 
ment through gradual changes in structure. 
Geological Distribution. It is doubtless true that 
the present distribution of a species has resulted from 
the conditions of its past distribution, even to the 
remotest times. Thus time becomes an important 
factor in the study of animal life. This is especially 
so if it be true that natural selection has operated in 
bringing about other changes, just as it has in altering 
the color of butterflies. Distribution in time is often 
called Geological Distribution. Indeed, to really 
understand an animal structure we need to study not 
only its present ways, but also the history of the 
struggles of its ancestors. In one way or another 
habits, structure, adaptations, and present and past 
distribution are so closely connected that while studying 
one we must study all. 
