310 ANNAL8 NEW YORK ACADEMY OF SCIENCES 



Associated with these great cycles of elevation and submergence are 

 climatic cycles from extremes of cold or arid zonar climates culminating 

 in glacial epochs, to the extremes of warm humid uiiilorm climates which 

 accompany or follow the extremes of submergence. 



The effect upon terrestrial life of progressive elevation of the land 

 areas, accompanied by a progressively cold climate at the poles and arid 

 climate in the interior of continents, wouTd be to adapt the terrestrial 

 life to cold, arid and highly variable climatic conditions. The environ- 

 ment favorable to tliis adaptation will appear first near the poles, and the 

 northern and southern fauna) will be more progressive and will tend to 

 disperse towards the equatorial regions. The wider area of emerged con- 

 tinents will tend to expansive evolution of the land faunas, and their 

 union into a single land mass will facilitate cosmopolitan distribution. 

 Owing to the conformation of the continents the dispersal will be chiefly 

 from the Holarctic region, the Antarctic and southern lands being unfa- 

 vorably situated for the evolution and dispersal of dominant races and 

 contributing but little to the cosmopolitan faunas of the emergent phase. 

 These conditions are also favorable to the development of higher, more 

 active and more adaptable types of terrestrial life, which tend to supplant 

 even in moist tropical regions the less adaptable remnants of the tropical 

 faunae which find there their last refuge. 



During the opposite phase of the cycle, the faunae become progressively 

 readapted to the moist tropical climatic environment. But owing to the 

 higher evolutionary stage acquired during the arid phase, the higher and 

 dominant types of the new fauna are evolved chiefiy by readaptation of 

 the dominant types of the arid phase and only subordinately by expansive 

 evolution of the tropical fauna surviving through that phase. 



The paleontologic record appears to be in exact harmony with these 

 principles, provided due allowance be made for its imperfections. The 

 geographic distribution of animals and plants affords far more complete 

 data, but their true significance has in my opinion been misinterpreted 

 by many zoogeographers. When interpreted in harmony with the prin- 

 ciples of dispersal shown to be true among mammals, they yield fully 

 concordant results. The geologic record is to-day far more incomplete 

 than is generally admitted, and will always be incomplete. Negative evi- 

 dence, while sometimes of high value, is more often worthless and should 

 never be admitted without a careful canvass of the situation in each 

 instance. 



The population of oceanic islands is notably incomplete and cannot be 

 interpreted as due to continental connection. The difficulties in the way 

 of over-sea transportation are best explained by the hypothesis of natural 



