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 



