30 THE DESCENT OF THE PRIMATES 



older orders, that flourished in the earliest Meso- 

 zoic, and in the Palseozoic period, such as the 

 Theromorpha, we ought to pause before affirming 

 that they too were already Amniota. 



On the other hand, the Palaeozoic Stego- 

 cephala, which are classed with the Amphibians, 

 might for that reason be said to be as yet de- 

 prived of an amnion. Nothing however prevents 

 us from assuming that in the period of the earth's 

 history in which this class flourished, the earliest 

 traces of this embryonic structure first originated. 

 In fact, its appearance is understood to have been 

 largely influenced by the formidable changes of 

 habit and of structure which must have come 

 about at the time the aquatic Vertebrates grad- 

 ually adopted, first, a semiterrestrial (amphibian) 

 existence, and then became specialized in differ- 

 ent classes of terrestrial animals. The majority 

 of the aquatic Vertebrates may then, as now, have 

 been oviparous. With the change towards a ter- 

 restrial existence, the eggs may yet for a long 

 time have continued to be deposited in the water 

 by many of them. Others may have adopted the 

 most various devices for the hatching and the 

 protection of their eggs, as is still faintly echoed 

 in those not numerous genera of frogs and toads 

 which carry and hatch their eggs, now on their 



