832 LIFE : OUTLINES OF GENERAL BIOLOGY 



the spur that led to the evolution of Reptiles from Amphibians. For 

 water basins were scarcer, and it became more difficult to have 

 giU-breathing larval stages living an aquatic life. Reptiles arose in 

 an age of aridity, and some of the fossils are betwixt-and-between 

 forms, in regard to which it is difficult to decide whether they should 

 be called Reptilian or Amphibian. 



But in the Permian Period another great change came about, 

 especially in the Southern Hemisphere, namely glaciation. It is 

 believed that this was more intense and extensive in the South than 

 were the recent (Pleistocene) Ice Ages, during which, in spite of 

 terrific elimination among animals, tentative men were able to get 

 a footing. So, millions of years previously, the Permian glaciation 

 may have been a spur to the evolution of Dinosaurs. 



Widespread drought would intensify the struggle for existence, 

 and would put a premium on swiftness of locomotion; and this in 

 turn would imply a rise of body-temperature owing to the stronger 

 development of heat-producing muscles. But the setting in of intense 

 cold would raise the new difficulty of conserving sufficient heat to 

 keep vital processes agoing. For it must be remembered that Reptiles 

 have not risen above cold-bloodedness, that is to say, the approxi- 

 mation of internal to external temperature. Only in Birds and 

 Mammals do we find warm-bloodedness, that is to say, the power of 

 conserving a constant body-temperature, day and night, year in 

 and year out. 



Thus we have the puzzle of the Dinosaurs evolving at a time when 

 there was great aridity in some areas, and great cold in others, and 

 when there were still more difficult places where both conditions 

 obtained. Perhaps the muscular development and activity of Dino- 

 saurs gave them a heat-supply large enough to do something to 

 compensate for the loss to the cold air ; perhaps the Dinosaurian 

 muscle produced more heat than that of modern reptiles, for "all 

 flesh is not the same flesh", and one recalls that even the tunny-fish 

 has considerable bodily warmth ; perhaps the external armature of 

 scales and scutes, sometimes very substantial, helped to conserve 

 the precious animal heat; perhaps some of the Dinosaurs lay low 

 when the worst came to the worst, becoming comatose in winter as 

 many living reptiles do. There are many perhapses, and the riddle 

 is unread; but we must think of the Dinosaurs establishing them- 

 selves in a world which was in part dismally arid and in part glacially 

 cold. Prof. R. S. Lull makes an interesting suggestion: "Possibly the 

 early dinosaur-like forms which dwelt within the influence of the 

 Permian cold became the birds, while those beyond its influence 

 remained dinosaurs, and as such were destined to dominate the 

 lands as no creatures before them had ever done." And another of 

 this thought-provoking palaeontologist's suggestions is the influence 

 of aridity in evolving bipedal progression, which many Dinosaurs 



