xiv. 2 (370 



2. Reptilia 



Towards the end of the Devonian period, say 350 million years 

 ago, the vertebrate organization produced a population of amphibian 

 creatures and from this has been derived not only various modern 

 groups classed as amphibia but also the more fully terrestrial popula- 

 tions that do not need to breed in water — the Amniota. Since that 

 time many divergent lines have evolved from this stock, including the 

 birds and the mammals, and it is evident, therefore, that it is likely to 

 be difficult to specify what is meant by a reptile, as distinct from an 

 amphibian or a bird or mammal. The term does not define a single 

 vertical line of development or branch of an evolutionary tree, but 

 is rather a horizontal division, marking a band on the evolutionary 

 bush, specifying a level of organization beyond that of an amphibian 

 but before that of either bird or mammal. Attempts have been made 

 to divide the reptiles vertically into sauropsidan (bird-like) and 

 theropsidan (mammal-like) lines, but such a division, although it has 

 some foundation, obscures the fact that their bush-like evolutionary 

 radiation has produced not two but many types. 



The existing reptiles belong to four out of the dozen or more main 

 lines that have existed. The most successful modern forms are placed 

 in the order Squamata, the lizards and snakes, the latter being of 

 relatively recent appearance in their present state. Secondly, the 

 tuatara, Sphenodon, of New Zealand is a relic surviving with little 

 change from the Triassic beginnings of this group. Thirdly, the 

 crocodiles are an older offshoot from the stock from which the modern 

 birds were derived. Finally the tortoises and turtles (Chelonia) have 

 retained in some respects the organization of still earlier times, 

 perhaps through the special protection of their shells. Though they 

 are much modified in some ways, they still show us several character- 

 istics of the earliest Permian reptiles. 



These four modern types are all that remain of the reptiles that 

 flourished throughout the Mesozoic, culminating in the giant dino- 

 saurs of the Jurassic and Cretaceous. Evidently a profound change 

 affected the population of the world, including the sea, between the end 

 of the Cretaceous and the Eocene. This change will be discussed 

 further in Chapter XXI, but we must briefly discuss here the possible 

 relation of the decline of the reptile populations to the rise of their 

 descendants, the birds and mammals. It can hardly have been only 

 the more efficient organization due to the warm blood that gave 

 these their opportunity, for there were forms in the Trias so similar 



