314 
KIRTLEY F. MATHER 
Episode number three was somewhat similar to that which had 
opened this chapter of earth history. Its chief event was the 
building of the Appalachian Mountains, which in their youth 
were mighty ranges not unlike the Rockies of British Columbia. 
The areas of hospitably shallow seas were still further reduced, 
and vast sandy plains stretched monotonously across the states 
of Texas, Oklahoma and New Mexico. Deserts, equalling the 
Sahara in magnitude and aridity, existed in parts of Europe and 
North America where today are fertile farms and majestic 
forests. The tribulations heaped upon the creatures of the land 
became even more numerous than before, as the humid areas of 
temperate climate were still further diminished in extent and 
number. The physiographic and climatic changes wrought 
during these three episodes in earth history could not fail to 
leave their indelible imprint upon the entire population of the 
globe, composed as it has always been of individuals quick to 
respond to modifications in their geographic environment. 
With the crumpling of the Appalachians, the mighty forces 
pent within the body of the earth seem to have spent their 
energy. Equilibrium of the opposing stresses was once more 
attained. The close of the Paleozoic Era was followed by another 
long era of comparative stability of the earth ^s crust. .Gradu- 
ally the normal temperate climate of the globe was restored; 
once more, broad shallow seas crept over the lower portions of 
the continental platforms; again, the regions of land and sea 
most hospitable to life expanded until they covered a large pro- 
portion of the planet; a long era of prosperity for the earth 
inhabitants was ushered in. This, the Mesozoic Era, the time 
of medieval life, is referred to as the Age of Reptiles, because 
during it the animals of this class aspired to the rulership of 
every domain of life, the land, the sea and the air. 
What were the effects of these revolutionary episodes upon 
the creatures subjected to them? Were they for good or for 
evil? Space forbids a full discussion of the problem; we may 
concentrate our attention upon a single class of animals and 
take their experiences as typical of all. And I choose from 
among the many available illustrations that afforded by these 
