130 FROM FISH TO PHILOSOPHER 



of the nature of the reptilian egg it is probable that even 

 these bred away from the water, and laid their eggs in 

 warm crannies in the sand. One may assume that many 

 of the Mesozoic reptiles may have spent the greater part 

 of their Hves away from watercourses, as do the arid- 

 living reptiles today, returning to the lush valleys as a 

 matter of convenience rather than necessity. As judged 

 by the surviving members of the order, most of the ter- 

 restrial forms were able to Hve almost independently of 

 a free water supply by virtue of their amniotic egg, their 

 impermeable skin, and their uric acid habitus. The rep- 

 tiles were deficient in only one notable respect— brains. 

 If Tyrannosaurus rex had the smallest brain per unit of 

 body weight of any vertebrate, it did not need a larger 

 brain in order to survive; it was so well adapted to its 

 world that lack of cimning did not count heavily against 

 it. But it was otherwise with the small and poorly 

 adapted mammals that Tyrannosaurus unwittingly 

 trampled underfoot: in these mammals the evolution of 

 the brain was to become of paramount importance. 



