130 DARWINISM AND THE PROBLEMS OF LIFE 



their extinction, but must not forget that they might 

 be converted into smaller types of animals ; if, that 

 is to say, the change in the vegetation took place 

 gradually, and the smallest individuals, or those that 

 required least food, were constantly selected. Another 

 reason why they could not persist in their old size is 

 that the very thick skull they had may have been a 

 very good protection ; but it meant only a very small 

 brain. But a small brain would be quite insufficient 

 to keep up the vital energy required in the active 

 life on the steppes. 



The pterosauri seem to have been really extirpated, 

 not transformed, as we do not find their descendants 

 in the birds. These flying lizards had naked bodies, 

 and it is possible that a sudden lowering of the climate 

 might account for their disappearance. The rapid 

 setting-in of severe cold would not give them time 

 to protect themselves by developing feathers, as their 

 relatives, the birds, had already done. All reptiles are 

 sensitive to cold in a high degree, as we see very 

 clearly in the lizard, which only displays its full vitality 

 in the sun. Hence it is that the giant-reptiles can 

 only be maintained in warm countries, the tropical 

 zone alone harbouring the mightier specimens to-day. 



Thus a change of climate that alters a country may 

 either compel a species to change, or, if it sets in with 

 comparative swiftness, extinguish it altogether. But 

 there is a third contingency. The species in question 

 may migrate. In this way the species that lived in 

 Germany during the glacial period retired to the North 

 and to the Alps, where they are still to be found. 



