238 THE ORIGIN AND EVOLUTION OF LIFE 



inhabited the entire globe until the comparatively recent period 

 of extermination by man, who through the invention of tools 

 in Middle Pleistocene time, about 125,000 years ago, became 

 the destroyer of creation. 



Single Character Evolution and Physicochemical 

 Correlation 



The principal modes of evolution as we observe them among 

 the mammals are threefold, namely: 



I. The modes in which new characters first appear, whether 

 suddenly or gradually and continuously, whether accidentally 

 or according to some law. 



II. The modes in which characters change in proportion, 

 quantitatively or intensively, both as to form and color. 



III. The modes in which all the characters of an organism 

 respond to a change of environment and of individual habit. 



The key to the understanding of these three modes is to be 

 sought first in changes of food and in changes of the medium 

 in which the mammals move, whether on the earth, in the 

 water, or in the air. The complexity of the environmental 

 influence becomes like that of a lock with an unlimited number 

 of combinations, because the adaptations of the teeth to varied 

 forms of insectivorous, carnivorous, and herbivorous diet may 

 be similar among mammals living in widely different habitat 

 zones, while the adaptations of the locomotor apparatus, the 

 limbs and feet, to the primary arboreal zone may radiate 

 into structures suited to any one of the remaining ten life 

 zones. Thus there is invariably a double adaptive and inde- 

 pendent radiation of the teeth to food and of the limbs to pro- 

 gression, and therefore two series of organs are evolving. For 

 example, there always arises a more or less close analogy be- 

 tween the teeth of all insect-eating mammals, irrespective of 



