CHAPTER XXX 

 ANATOMICAL STRUCTURE AND CLIMATIC EVOLUTION 



General views in regard to the ancient climatic conditions 

 must first occupy attention in the present connection. It is 

 commonly conceded, on the basis both of the nature of ancient 

 organisms and of the evidence supplied by geologic strata, that 

 the earth was formerly much warmer than it is in the present 

 epoch. It is further clear that, other things being equal, the 

 greatest degree of warmth existed in the most remote past. This 

 general situation, however, does not exclude the recurrence at 

 long intervals of periods of refrigeration or glaciation. An age of 

 ice is known to have occurred, not only at the end of the Cenozoic 

 as originally established by Agassiz, but glacial periods also termi- 

 nated both the Mesozoic and the Paleozoic as ordinarily denned. 

 Evidence of still earlier glacial epochs which exercised a devastating 

 influence on the most ancient animal and plant populations of 

 our earth is not lacking. 



Evidence in regard to glaciation in former epochs is both direct 

 and indirect. Direct testimony concerning former ages of ice 

 is supplied by the comparative study of deposits formed in con- 

 nection with the existing glaciers of high latitudes or of high 

 altitudes. Intimately connected with glacial phenomena are the 

 formations of clays, till, and coarse morainal matter resulting 

 from the movement and melting of ice. Indications of the kinds 

 just enumerated in earlier geological strata supply direct testimony 

 as to former glacial action. Indirect information in the same 

 direction is often furnished by the wholesale extinction of impor- 

 tant groups of plants and animals. For example, in the Permian 

 glaciation which marked the close of the Paleozoic the treelike 

 cryptogams, which have contributed so largely to the formation 

 of the older deposits of combustible minerals, disappeared entirely 

 as an important constituent of the plant population of our earth. 

 Glacial epochs are, however, not of direct importance in relation 



417 



