248 EVOLUTION AND GENETICS 



Ultimately, as species give rise to other species, this change of 

 response must find expression in change of structure and function 

 as heritable properties. 



Many recognizable forces may contribute to the initial shift 

 of an organism from one environment to another. Change of 

 climate due to geological factors might readily cause readjustment. 

 Change of food supply, involving either climatic factors and soil 

 conditions, or interspecific relationships probably occurs even 

 more often than marked geological changes. Overpopulation is 

 commonly regarded as a potent factor in evolution. In connection 

 with all three forces, an animal's power of migration may deter- 

 mine the nature of its response, and in some cases it is probably a 

 primary factor in itself. 



Change of Climate. Geology and paleontology together show 

 that from tiine to time in the remote past climatic conditions 

 have varied greatly. Our own continent has passed through 

 tropical and arctic conditions, as well as temperate periods with 

 climates like that of the present. "Geologists now know of seven 

 periods of decided temperature changes (earliest and latest Late 

 Proterozoic, Silurian, Permian, Triassic, Cretaceous-Eocene, and 

 Pleistocene) and of these at least four were glacial climates (both 

 Proterozoic times, Permian and Pleistocene) . The greatest inten- 

 sity of these reduced temperatures varied between the hemispheres, 

 for in the earliest Late Proterozoic and Pleistocene it lay in the 

 northern, while in latest Proterozoic and Permian time it was 

 more equatorial than boreal. Cooled climates occur when the 

 lands are largest and most emergent, during the closing stages 

 of periods and eras, and cold climates nearly always exist during 

 or immediately following revolutions, when the earth is undergoing 

 marked m^ountain making " (Schuchert) (Fig. 149). 



Factors Which Influence Climate. The things which modify 

 temperature may also be instrumental in determining the amount 

 of precipitation, the relation of seasons, and minor fluctuations. 

 A high range of mountains, for example, may cause abundant 

 rainfall on one side and desert conditions on the other by cooUng 

 moisture laden currents of air as they blow toward it from the 

 nearest ocean. This is true of mountains of the western United 

 States. The same effect may be responsible for localization of 

 rainfall. High altitudes are also subject to fluctuations of many 

 degrees in temperature between day and night, so that organisms 



