43 o REPTILES xv. 19 



probably as great a biomass spread over a large number of small in- 

 dividuals. Large size in a reptile may help to conserve heat (p. 372), 

 but could also endanger the animal from overheating, since the ratio 

 of surface area to volume decreases as the absolute size increases, and 

 heat cannot be lost so readily through the skin. Up to a point size may 

 be a protection, but it involves the dangers of those who place all the 

 eggs in one basket ; incidentally, the actual eggs of these large animals 

 must have provided formidable physical problems for their support. 



Parallel evolution of several lines descended from a single stock is 

 as common among reptiles as among other groups of vertebrates. 

 Thus the bipedal habit, with hind legs longer than the front, has been 

 adopted independently by a number of diapsids; again, elongated jaws 

 are found among fish-eaters, whether ichthyosaurs, plesiosaurs, phyto- 

 saurs, crocodiles, or mosasaurs. 



Although it is difficult to see in all this any persistent tendency 

 except to change, yet the very fact that each type is so rapidly replaced 

 suggests that descendants in some way more efficient are continually 

 appearing. In the case of the reptiles the more interesting of these 

 are the birds and mammals, and we shall therefore leave the problem 

 of serial replacement among amniotes for later discussion. Meanwhile 

 we may note once again that the reptiles surviving today, although not 

 of larger size nor obviously better suited for life than their mesozoic 

 ancestors, yet exist in considerable numbers alongside and even in 

 competition with the birds and the mammals. 



