No. 624] MIGRATION A FACTOR IN EVOLUTION 



65 



stratum, are subjecting many animals to stress, and to 

 the process of acclimatization to the high altitude con- 

 ditions. The climatic diversity shown vertically, also ex- 

 presses itself longitudinally, by interference with free 

 circulation of temperature, moisture, and other climatic 

 factors, and tends to produce the varied climatic zones, 

 such as tropical, temperate, and polar, as well as the 

 diversity due to humidity. These diversities mean that 

 many minor circulating systems are caused and conse- 

 quently there are varied local wind systems, further 

 favoring diversity. All of these influences tend to favor 

 local or relatively limited migration, rather than the 

 widespread dispersal of animals subjected to such con- 

 ditions. 



The hydrosphere is influenced similarly. The diversity 

 of the lands favors diversity in inland waters, and the 

 arid climates favor saline waters. Inland waters under 

 these conditions tend to be isolated and varied. In the 

 sea the deepened waters produce stresses similar to 

 those on land produced by altitude, and the elevations 

 and relative increase in the land area interferes with the 

 circulation of marine waters and favors local differences 

 and local stresses. AVith the deepening of the seas, the 

 shallow waters are restricted and the littoral animals are 

 under increased stress. It is seen, therefore, that the 

 conditions dominant on a world scale are those of stress 

 or strain, now in the process of adjustment. It should 

 be observed that all spheres, the atmosphere, hydro- 

 sphere, lithosphere, and the biosphere are involved in the 

 same general interacting process. It is only by viewing 

 the subject broadlv that Ave thus gain this ])ei-s|»ect ive of 

 the status of our own times. 



We .nay mnv tiini to eertain .letails which will help to 



tioH. Tlu' same ,uro.i|.in- <.r iiitlne.KM^s will be used which 

 [a) Afmosphrric I u/hwucrs.^-The atmospheric factors 



