VARIATION BY ENVIRONMENT. 
247 
Fig. 117. — Animals of Galapagos islands. 
lime, tlie geological ages differed greatly in environ¬ 
ment. Blit tliese differences in general were brought 
about very gradually, and when they came about 
more or less suddenly, as was the case with the 
Glacial epoch, life did not have sufficient time to 
Environment is slowly but constantly changing. 
As we have seen from the early chapter of this vol- 
