372 REPTILES xiv. z- 



to mammals in their skeletons that we may reasonably (though 

 not certainly) suppose them to have been warm-blooded. There were 

 birds with feathers in the Jurassic, and it is probable that they also 

 already had warm blood. However, as a working hypothesis, we may 

 suppose that the climate, which had been suitable for reptiles in the 

 Mesozoic, became less so in the early Tertiary, and the most obvious 

 suggestion is that colder conditions developed all over the earth's 

 surface. The modern reptiles for the most part live in the temperate 

 and tropical zones, indeed they flourish only in the latter. However, it 

 must be remembered that climate fluctuates continually (p. 13); it is 

 dangerous to make generalizations about conditions over such long 

 periods as the Cretaceous. 



3. The organization of reptiles 



The organization we call reptilian is, generally speaking, suitable for 

 life in warm countries, though two species, the common lizard 

 (Lacerta vivipara) and the adder ( Vipera berus), are found as far north 

 as the Arctic Circle. No doubt the distribution of reptiles is limited 

 largely by the fact that they cannot maintain a temperature above that 

 of the surroundings by production of heat from within. The wide- 

 spread idea that reptiles have no means of regulating their body 

 temperature, however, has been overemphasized. Bogert and his 

 collaborators in the U.S.A. have shown that in the wild (though not 

 as a rule under laboratory conditions) reptiles are often able by suitable 

 behaviour to maintain their body temperatures at a remarkably high 

 and constant level throughout much of the day, by varying their 

 exposure to the available sources of heat. When they get cold they 

 bask in the sun or rest on warm rocks; when they get too hot they 

 shelter under vegetation or in holes. In some species, too, colour 

 change plays a part in temperature control, the animals becoming 

 darker or lighter in colour, according to whether heat absorption or 

 reflection is the appropriate response. 



It has also been shown that each species of reptile has an optimum 

 range of temperature, below which the animals become inactive and 

 above which they quickly die. In some desert lizards the upper limit is 

 above 40 C. The range tends, as one would expect, to be higher in 

 diurnal than nocturnal forms, and is in general higher in lizards than 

 it is in snakes or alligators. 



The reptilian method of temperature control differs essentially 

 from that of mammals in that it depends on the availability of external 

 sources of heat such as the sun, rather than on the ability to conserve 



