CHAPTER VI 

 FACTORS AFFECTING ANIMAL LIFE 



VARIATIONS in the several different types of ani- 

 mal life from place to place on the earth's 

 surface or in the oceans, and from one geolog- 

 ical epoch to another, are directly or indirectly brought 

 about as a response to variations in the factors which 

 bear more or less directly upon the animals involved. 



Frogs are not found in deserts, nor are there any 

 lizards in cold regions. Elephants, rhinoceroses and 

 tapirs now live only in restricted areas in the tropics, 

 but in the Pleistocene all three ranged far to the 

 northward of their present habitats. On the New 

 Siberian Islands in the Arctic Ocean the bones of 

 Arctic elephants or mammoths are to be found in 

 great abundance. 



So before we can discuss the problem of the changes 

 in and the development of animal forms it is essential 

 that we understand just what the most important of 

 these factors are. 



The chief factors affecting all living things are the 

 great diversity in the form in which the supply of 

 necessary new materials is offered, and the great 

 diversity in the chemical and physical environment 

 or surroundings in which they must be taken up 

 and used. 



Only the plants containing chlorophyll or some 

 similar substance are able to build up organic out of 



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