THE VOYAGE OF EVOLUTION 113 



For our present purpose another change is far more important. 

 At this time apparently there occurred one of the most vital steps 

 in the evolution of our direct ancestors, the mammals. Extreme 

 aridity and low temperature were both characteristic of certain 

 epochs of the Permian Period. Among the more progressive types 

 of land animals aridity has a tendency to accelerate development. 

 It places a premium upon the power to travel, and especially upon 

 speed. As Lull puts it: "Not only are food and water scarce 

 and far between, but the strife between pursuer and pursued 

 becomes intensified — neither can afford to be out-distanced by the 

 other. This means increased metabolism, which in turn generally 

 implies not only greater motive powers but higher temperature. 

 With increasing cold a premium would be placed upon such crea- 

 tures as could maintain their activity beyond the limits of short- 

 ening summers, and this could be accomplished only by the 

 development of some mechanism whereby a relatively constant 

 temperature could be maintained within the animal regardless of 

 external conditions." In other words, there arose warm-blooded 

 animals whose temperature was more or less independent of the 

 surrounding air instead of varying with it as is the case in cold- 

 blooded animals. Among mammals this led to the production of 

 the young within the body of the mother instead of from eggs 

 in which the mother took little or no interest after they were laid. 

 Among birds it forced the mother to care for the eggs if they were 

 to be hatched. Thus the relation of mother and child became 

 firmly established. The later development of this relation has been 

 the chief source of all that is best in mankind. 

 K We must not suppose that during the climatic stress of the 

 * Permian the mammals emerged full-fledged. The first warm- 

 blooded creatures were not warm in the sense that we are. Their 

 temperature varied through wide limits, just as now is still true 

 of infants even when they are perfectly healthy. Nor were the 

 young born alive at first, for eggs continued to be laid as is 

 today the case with some of the most primitive mammals of 



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