254 THE ORIGIN AND EVOLUTION OF LIFE 



world; consequently the vicissitudes of the physical environ- 

 ment as causes of the vicissitudes of the life environment of 

 mammals afford the most complex examples of interlocking 

 which we know of in the whole animal world. In other words, 

 the mammals interlock in relation to all the surviving forms of 

 the life which evolved on the earth before them. Although 

 suggested nearly a century ago by Lyell, the demonstration is 

 comparatively recent that one of the principal causes of the 

 extinction of certain highly adaptive groups of mammals is 

 their non-immunity to the infections spread by Bacteria and 

 Protozoa.' Thus a change of environment and of climate may 

 not affect a mammal directly but may profoundly affect it in- 

 directly through insect life. 



These closely interlocking relations of the mammals with 

 their physicochemical environment and their life environment 

 have been subject to constant disturbances through the geo- 

 logic and geographic shifting of the twelve or more habitat 

 zones which they occupy. Yet the earth changes during the 

 Tertiary, the era during which mammalian evolution mainly 

 took place, were less extreme than those during Mesozoic and 

 Palaeozoic time. This is because the trend of development of 

 the earth's surface and of its climate during the past 3,000,000 

 years has been toward continental stability and lowering of 

 general temperature in both the northern and southern hemi- 

 spheres, terminating in the geologically sudden advent of the 

 Glacial Epoch, with its alternating periods of moisture and 

 aridity, cold and heat, which exerted the most profound influ- 

 ence upon the food supply, insect barriers, and other causes 

 affecting the migrations of the Mammalia. These causes com- 

 pletely change the general aspect of the mammalian world in 



^ For the history and discussion of this entire subject see Osborn, H. F.: "The Causes 

 of Extinction of Mammalia," Amcr. Naturalist, vol. XL, November and December, 

 1906, pp. 769-795, 829-859. 



