ZOOGEOGRAPHY 305 



Dependence Upon Environmental Conditions. — Another factor of 

 distribution is the dependence of each kind of animal upon certain condi- 

 tions. This is discussed in the chapter on ecology, but it should be 

 pointed out Here that since animals are physiologically and often mor- 

 phologically deoendent upon certain conditions, the absence of these 

 conditions will limit the range of the species in nature; or, as is often 

 said, conditions other than those to which the animal is adjusted con- 

 stitute a barrier to migration. 



Barriers to migration are of two kinds: first, barriers that cannot be 

 passed by means of the powers of locomotion of the animal, and second, 

 uninhabitable areas. An expanse of salt water, such as intervenes be- 

 tween islands and the mainland, is a barrier to manj'' land animals because 

 they cannot swim across it, while a plains area is a barrier to typical 

 forest forms, such as flying squirrels, not because they cannot move about 

 on the plains but because the conditions of life are so unfavorable for 

 existence that the forms cannot spread across them. The nature and 

 effectiveness of barriers can easily be shown to depend upon the nature of 

 the animal. Conditions which serve as a barrier to some animals will 

 not act as a barrier to others. Furthermore, any feature of the environ- 

 ment, whether temperature, moisture, size of area, soil, absence of food, 

 vegetation, other animals, or any other condition, may be the specific 

 cause of unfavorableness if the animal is closely dependent upon some 

 quality or degree of that feature. 



Diversity and Mutability of Environment. — Closely related to migra- 

 tion as a factor of distribution and to the physiological and morpholog- 

 ical dependence of animals on specific conditions is the diversity and 

 changeableness of the earth's surface. It is common knowledge that 

 there are great differences in temperature between the equatorial regions 

 on the one hand and the polar regions on the other; that there are great 

 differences in temperature between places in the same latitudes, depend- 

 ing upon altitude and upon the presence of large bodies of water, ocean 

 currents, etc.; that the amount of rainfall varies enormouslj'', being in 

 some places as high as 100 inches annually and in others as little as a 

 trace for years at a time; that there are great differences in soil; that some 

 regions have only a very scanty desert flora, others are open grass lands, 

 and still others are covered with forests which varj'' in density and the 

 species which compose them ; and that the waters vary in chemical com- 

 position, depth, size, etc., in addition to temperature. In fact there are 

 no two areas that are exactly the same in the conditions which may 

 affect animals, and no single area, if at all extensive, is the same 

 throughout. 



The data of geology also reveal that the physical conditions on the 

 surface of the earth are constantly being altered, and that the changes 

 which have taken place in the past are very great (Figs. 243 and 244). 



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