136 FROM FISH TO PBDDLOSOPHER 



Along the mammalian line, however, the Therapsids, 

 retaining the urea habitus of the nascent amniotes, 

 adapted to Permian frigidity to give rise to the hetero- 

 thermic theriodonts and later mammaUan forms. 



It is scarcely worth arguing how much biological ad- 

 vantage over the reptiles the birds derived from their 

 warm-blooded state; mere preponderance of numbers in 

 respect to families, genera, and species does not prove 

 the case for biological advantage, any more than does 

 the ability to soar in the air, and in the long view the 

 birds flew into a blind alley when they took to flight. 

 What they might have gained by their warm-blooded- 

 ness they lost by abandoning their foreUmbs to the role 

 of wings. But that warm-bloodedness worked ultimately 

 to the great advantage of the mammals is not to be 

 denied: they were earthbound, it is true, but this cir- 

 cumstance rather than handicapping them was ulti- 

 mately to profit them— because, to satisfy the hunger 

 that was now greatly enhanced and constantly sustained 

 by the demands of body-temperature regulation, they 

 were impelled to remain active for long periods through- 

 out the year. When winter came the birds could fly to 

 warmer zones and the reptiles could sink into the obhv- 

 ion of cold sleep; but the mammals had to stick it out 

 where they were and continue to eat and drink to main- 

 tain the constancy of their internal environment. It was 

 simply a case of keep active and awake— or else! 



Throughout the Mesozoic the primitive mammals 

 made httle progress, possibly because grasses, cereals 

 and fruits were still rare, and competition from the car- 

 nivorous dinosaurs was keen. Their opportunity did not 

 come until the Laramide revolution and the opening of 

 the Cenozoic (kainos = recent; zoe = life) era, when 

 they underwent rapid multiphcation and transformation 

 in modem forms. 



The whole of the Cenozoic, from the end of the Cre- 

 taceous to what the geologist calls 'Recent' time (which 



