178 ANNALS NEW YORK ACADEMY OF I^CIEXCES 



emergence oi' the coiitiiu'iits to be outward iroiu tlie two great northerly 

 masses, and especially i'rom Asia. The tropical and southern continents 

 would be the refuge of the less adaptable and progressive types. 



This phase of climate should, therefore, favor a liigher development 

 and greater activity of land life, while the geographic conditions favor 

 cosmopolitan faunae. When the climatic pendulum began to reverse its 

 swing, the continents became isolated and fheir fauna.^ developed inde- 

 pendently; but the dominant animals of these fauna? wlien first isolated 

 would be those previously develoi)ed during the arid phase, and these 

 would readapt themselves to the new conditions of moist and uniform 

 climate, of prevalent forest and swamp and of abundant food. 



COMPARISON WITH TllK I'AI.EONTOLOGKAL UKCOIM) 



How far do these a priori deductions correspond with the facts, as 

 obtained from the geological record ? Tn the first place, we should keep 

 in mind that our record of the land life of the emergence phases is very 

 defective. The sediments of this phase, where deposited along the con- 

 tinental margins, are limited in area, thick and very barren, the condi- 

 tions of their deposition being generally unfavorable to the preservation 

 of fossils. The sediments of the interior of the continents, river and 

 floodplain deposits of the Cenozoic era are more widespread and furnish 

 an extensive record of Tertiary and Quaternary land life; but those of 

 the preceding periods of aridity have been re-eroded and carried down 

 to the marginal and littoral areas during the period that has elapsed 

 since they were first deposited. Of the pre-Tertiary epicontinental de- 

 posits, only the coast margin, littoral and marine deposits are extensively 

 preserved. That means that the record of Mesozoic and Paleozoic land 

 life as preserved to us is chiefly the record of the coast-swamp and low- 

 land regions and that we know nothing of the life of the upland, except 

 by a rare accidental preservation. Tu considering the evidences of cli- 

 matic adaptation during the Mesozoic, this must be kept clearly in mind. 



The great mass of evidence in favor of adaptation to jn-ogressively 

 arid climate and of dispersal from the northei-ii land regions is derived 

 from the recorded history of the Mammalia during the Tertiary and 

 Quaternary and from comparison of their former and present geographi- 

 cal distribution. It has long been recognized that the present distribu- 

 tion of mammals is due chiefly to migration from the great northern 

 land mass, and the connection of this southward march with progressive 

 refrigeration in the polar regions was made more than a century ago 



