PHYSICAL AND CLIMATIC FACTORS 419 



animals dominate over the poikilothermous by reason of their 

 constant internal temperature. It follows that if a region of 

 high temperature, populated by poikilothermous and homo- 

 thermous animals, were to undergo a reduction of temperature 

 (as by greater elevation of the land above sea-level or the 

 approach of an ice-age), the homothermous animals would 

 survive, whereas the poikilothermous forms would be very 

 likely to go extinct, especially if they were of large size. This 

 may be what happened at the cold end of the warm secondary 

 era (Trias to Cretaceous inclusive, the " age of reptiles "), 

 when the reptiles all but went extinct, and were only survived 

 by the present-day forms, which furnish a miserable sample 

 of former richness of the reptilian fauna. At the same time, 

 the homothermous birds and mammals survived, the latter to 

 become the dominant animals. 



It is seen, therefore, that certain of the greatest episodes 

 in the history of the vertebrates, such as the evolution of the 

 amphibia, may have been largely conditioned by climatic 

 changes in the earth's crust. Other episodes were probably 

 related to adaptations to more fixed climatic conditions. Of 

 these, two only will be mentioned here. The first concerns 

 the evolution of the early fish. The original ancestors of the 

 chordates must have been marine forms, but there are certain 

 considerations which suggest that the evolution of the early 

 chordates took place in fresh or estuarine water. The typical 

 chordate method of locomotion by undulations of the body 

 from side to side may be regarded as an adaptation to life in 

 rivers in which there is a more or less constant flow of water 

 in a certain direction. 



The other episode concerns the evolution of man, part of 

 whose ancestral history is related to the habit of living in 

 trees. It is common for arboreal animals to retain unspecialised 

 limbs, and to acquire the capacity of opposing one or more 

 digits to the others, and so be able to grasp branches firmly. 

 At the same time the sense of smell becomes less important, 

 while that of sight becomes dominant, leading to binocular and 

 stereoscopic vision, and the capacity to estimate distance. This 

 is of importance to an arboreal animal in estimating the strength 



