EVOLUTION, VARIATION AND HEREDITY 



475 



various conditions of life, and, finally, to become a dominant class 

 culminating in Man himself. Lack of space prohibits more than 

 this brief indication of their line of origin. 



To pass back to the second group of primitive reptiles, we 

 find in early Permian times a form termed Seymouria. This is a 

 slightly stouter animal than Eryops, exhibiting many Stegocephalian 

 features, but showing a certain approach towards the higher 

 Reptilia. It may be considered as a lowly representative of a 

 diversified group of primitive Reptiles, the Cotylosauria, from which 



FIG. 1 68. Ship-lizard, Edaphosaurus cruciber, Per mo-Carboniferous, 

 North America. -After Case. 



the higher forms originated, and which was duly established by the 

 end of the lower Permian. At any rate, three, or perhaps four, 

 distinct lines of development from this group can be traced. The 

 Cotylosaurs were land dwellers, but from time to time some of their 

 descendants returned again to the sea. The first line we shall 

 notice is but little known, and culminated in the Turtles and Tortoises, 

 or Chelonia, as they are collectively termed. They first make their 

 appearance in early Triassic times as quite highly specialised forms, 

 easily recognisable as similar to their descendants living to-day. 

 They are characterised by the broadening and flattening of their 

 ribs and the development of a characteristic box-like skeleton. 

 Of their previous history but little is known, save for an interesting 

 form, Eunotosaurus , from the late middle Permian beds, an animal 

 with broad expanded ribs and a peculiar skull that is regarded by 

 some authorities as a form linking the Chelonia with the Cotylosaurs. 

 The second line is the Plesiosauria, large aquatic lizards that make 

 their appearance in Mid Triassic times. Their remains are probably 

 the most widely spread and common of all fossils, being recorded 

 from all parts of the world. Of the hundreds of skeletons known 



