24 LAND MAMMALS IN THE WESTERN HEMISPHERE 



and for the earth's later ages, at least, a great deal may be 

 learned regarding the successive forms of the land-surfaces 

 in the various continents. It would be very desirable to ex- 

 plain the methods by which these results are reached, but this 

 could hardly be done without writing a treatise on physiog- 

 raphy, for which there is no room in this chapter. We must 

 be permitted to make use of the results of that science without 

 being called upon to proye their accuracy. 



No factor has a more profound effect in determining the 

 character and distribution of living things than climate, of 

 which the most important elements, for our purpose, are 

 temperature and moisture. One of the most surprising results 

 of geological study is the clear proof that almost all parts of 

 the earth have been subjected to great vicissitudes of climate, 

 and a brief account of the evidence which has led to this un- 

 looked for result will not be out of place here. 



The evidence of climatic changes is of two principal kinds, 

 (1) that derived from a study of the rocks themselves, and (2) 

 that given by the fossils of the various epochs. So far as the 

 rocks laid down in the sea are concerned, little has yet been 

 ascertained regarding the climatic conditions of their formation, 

 but the strata which were deposited on the land, or in some 

 body of water other than the sea, often give the most positive 

 and significant information concerning the circumstances of 

 climate which prevailed at the time of their formation. Cer- 

 tain deposits, such as gypsum and rock-salt, are accumulated 

 only in salt lakes, which, in turn, are demonstrative proof of 

 an arid climate. A salt lake could not exist in a region of 

 normal rainfall and, from the geographical distribution of such 

 salt-lake deposits, it may be shown that arid conditions have 

 prevailed in each of the continents and, not only once, but many 

 times. As a rule, such aridity of climate was relatively local 

 in extent, but sometimes it covered vast areas. For example, 

 in the Permian, the last of the Palaeozoic, and the Triassic, the 

 first of the Mesozoic periods (see Table, p. 15) nearly all the 



