122 PRIMARY FACTORS OF ORGANIC EVOLUTION. 



der of reptiles excepting in a few Lacertilia (Gecconi- 

 dae). They thus differ from the Mammalia, whose 

 characters are approached more nearly by some of the 

 terrestrial Dinosauria in this respect. Leaving this 

 order, we soon reach the prevalant ball-and-socket 

 type of the majority of Reptilia. This strong kind of 

 articulation is a need which accompanies the more 

 elongated column which itself results at first from the 

 posterior direction of the ilium. In the order with the 

 longest columrt, the Ophidia, a second articulation, 

 the zygosphen, is introduced. The mechanical value 

 of the later reptilian vertebral structure is obvious, and 

 in this respect the class may be said to present a higher 

 or more perfect condition than the Mammalia. 



In review it may be said of the reptilian line, that 

 it exhibits marked degeneracy in its skeletal structure 

 since the Permian epoch ; the exception to this state- 

 ment being in the nature of the articulations of the 

 vertebra?. And this specialization is an adaptation to 

 one of the conditions of degeneracy, viz., the weaken- 

 ing and final loss of the limbs and the arches to which 

 they are attached. 



The history of the development of the brain in the 

 Reptilia presents some interesting facts. In the dia- 

 dectid family of the Permian Cotylosauria it is smaller 

 than in a Boa constrictor, but larger than in some of 

 the Jurassic Dinosauria. Marsh has shown that some 

 of the latter possess brains with relatively very narrow 

 hemispheres, so that in this organ those gigantic rep- 

 tiles were degenerate, while the existing streptostyli- 

 cate orders have advanced beyond their Permian an- 

 cestors. 



There are many remarkable cases of what may now 

 be safely called degradation to be seen in the contents 



