EVOLUTION 977 



Period there was great humidity and much low-lying swampy 

 ground. These conditions favoured luxuriant vegetation: witness 

 the growth of the great cryptogamic forests associated with the 

 coal measures. Naturally enough this was a period during which 

 Amphibians had their Golden Age ; there was considerable diversity 

 of structure, and some types attained large size. If ever there was a 

 pulse in evolution, it was surely among these primeval Amphibians, 

 signalised by many new acquisitions, such as fingers and toes, 

 lungs and posterior nostrils, a mobile tongue and probably a voice. 



Similarly one might suggest that humid conditions during part 

 of the Jurassic Period led some of the big reptiles to give up hunting 

 and take to vegetarianism, thus becoming large of size and slow in 

 pace. Many of them sank gradually to extinction. But this state of 

 affairs may be contrasted with the conditions of aridity and steppe- 

 country which led at a later time to the emergence of birds. The 

 first known fossil bird, Archaeopteryx from the Upper Jurassic, 

 cannot have been by any means the actually first creature to 

 deserve being called avian ; and one can picture its ancestors evolving 

 from a Pseudosuchian or Dinosaurian reptile stock, adapted to 

 swift movement on dry ground, and to flying leaps into the shelter 

 of scrub-like bushes which grew sparsely on the bare tracts of an 

 unfriendly steppe-land. 



Perhaps it was a time of dry cold in the Triassic Period that 

 prompted some Dinosaurs to acquire habits of great activity and an 

 intense metabolism, to exchange scales for a non-conducting garment 

 of fur, and to effect an improvement in the linkages of nerve and 

 muscle and gland that led to the great acquisition of warm-blooded- 

 ness, which means being able to keep up a uniform body-tempera- 

 ture, day and night, year in, year out. In a word, aridity prompted 

 the evolution of mammals. 



At another time, however, when the conditions of climate and 

 surface-relief were different, there was a luxuriant vegetation of 

 juicy herbs and fruit-bearing trees, which would prompt another 

 kind of evolution, leading to browsing herds and to a wealth of 

 resident birds. Obviously one must not be content to say "aridity 

 did this" and "humidity did that"; one must become familiar with 

 the details now accumulating in regard to the history of climates, as 

 in Brooks's remarkable book Climate Through the Ages', and one 

 must then try to correlate particular climatic peculiarities with 

 particular uplifts and depressions in organic evolution. 



But what of the other side — the pulse from within the organism ? 

 It is well known that a particular tendency in an individual may 

 become cumulative and acquire organic momentum, as in the case 

 of muscle-forming or of fattening. A degenerative or disintegrative 

 change, apart from microbes, may also spread in the body and gain 

 in malign potency. So it may also be in racial evolution, that a 



VOL. II R 



