LIFE IN THE PAST 179 
lengthened one, while the toad has learned to crowd 
its tadpole life within a few weeks. It would seem 
as if, in the earlier times of the Mesozoic, this same 
change of habit had been going on. With the drying 
up of the swamp, some of the amphibians crowded 
their tadpole stage further and further back, until it 
was completely accomplished before their young left 
the egg. An examination of the development of the 
reptile in the egg will show a stage very similar to 
the fish and to the amphibians, but this is all experi- 
enced before the reptile emerges from the egg. The 
reptilian egg, unlike that of the frog, is covered with 
a shell, packed away under the surface of the ground, 
and left to its own fate. If, as most geologists be- 
lieve, the climate of the Mesozoic was distinctly warm, 
this habit of the parent of forsaking the egg was not 
a serious matter. However the creatures arose, it is 
certain that in this Mesozoic age reptiles roamed the 
forests, swam the seas, and even flew in the air. 
Probably at no other time in the earth’s history has 
any one class of animals so completely dominated the 
situation as did the reptiles of this age. They were 
not only abundant; they were frequently enormously 
large. Their skeletons are among the most interesting 
that we find to-day. Gigantic lizards, seventy feet 
long and eighteen feet high at the shoulders, dragged 
their heavy bodies through the marshy edges of the 
