EVOLUTION, VARIATION AND HEREDITY 483 



clawed fingers on the front of the wing, and jaws with a number of 

 pointed teeth. Later, in Comanchian times, we encounter another 

 bird group exemplified by Ichthyornis and Hesperornis. These were 

 fairly typical water birds in general appearance, had shortish tails, 

 and were covered with feathers. Their heads have certain reptilian 

 resemblances, the jaws bore teeth, and the breast bone was not keeled. 

 True birds appear in the Tertiary, and are closely related to living 

 forms. In spite or the breaks in the record, however, there is little 

 doubt about the general relationship of true birds with their precursors. 

 We have thus in a very superficial manner touched on some 

 of the main types of the Reptilia, indicated the important lines of 

 evolution they followed, and shown their relation to the animals 

 living to-day. Only a few of the striking forms have been mentioned 

 and many more are known. As a class they spread widely over the 

 surface of the earth, and were so plentiful and diverse in the Mesozoic 

 times that we often refer to this period as the " Age of Reptiles." 

 As to the reasons for the decline of the Reptiles, the times in which 

 they lived are so remote that they become matters of conjecture, 

 but certain points seem to stand out fairly clearly. The greater 

 part of the Mesozoic was a period of relative stability, or of only 

 very gradual change, so that many groups of the Reptiles were able 

 to branch out and become highly adapted to certain environments. 

 The close of the era was marked by very far-reaching and profound 

 changes in the configuration of the land surface. These were 

 probably accompanied also by considerable climatic alterations. 

 The result was, that the highly modified forms had, as the very 

 outcome of this specialisation, apparently lost the power to become 

 re-adapted to altered environmental conditions. Thus during this 

 transition time the Ichthyosaurs, Plesiosaurs, Mosasaurs, Ptero- 

 dactyls, and both groups of the Dinosaurs, died out in the Middle 

 and Upper Cretaceous times. This left a number of forms which 

 were either little specialised as the early Mammals, Crocodiles, and 

 Lacertilia, and capable of giving rise to new diversity, or else, like 

 the Aves and Chelonia, capable of becoming adapted to the changed 

 conditions. It seems probable, then, that the end of the dominance of 

 the Reptiles was the result, direct or indirect, of changed environment. 



The history of the Reptiles serves to illustrate the general 

 course of evolution in the animal kingdom in general. In the first 

 place, we have seen lowly little specialised forms branching out into 

 various modes of living, and becoming fitted for life in various 

 environments. That is to say, the generalised type spreads out 

 into a series of adaptive specialisations that produce a diversity of 

 forms, and we may say that, on the whole, groups of animals have 



