No. 624] MIGRATION A FACTOR IN EVOLUTION 



and lithosphere, and lie has indicated their modes of in- 

 teraction in a strictly dynamic manner. He shows that 

 during a period of land elevation and mountain forma- 

 tion, with cold, dry, diverse climatic differences and zonal 

 arrangements on land and with a deepening of the sea, 

 these conditions tend to change progressively toward a 

 moist, warm, uniform and tropical condition, which is 

 related to the land equilibrium developed during base- 

 level on land, and a marine condition of extensive shallow 

 seas. The process of adjustment to these strains beauti- 

 fully illustrates Bancroft's law. It is not an accident 

 that mountains are centers of origin and dispersal of ani- 

 mals, nor are they solely refuges where endemic forms 

 escape the competition of the lowlands. Mountain re- 

 gions in their elevation subject whole populations to 

 severe climatic and other stresses of many kinds, depend- 

 ing on the physical and vegetational diversity of the 

 region, and doubtless thus many animals become extinct, 

 while others as individuals or as a race become acclimated 

 to the new and changed conditions and thus survive. 



It seems strange that, although dynamic principles are 

 shown in almost ideal form in the unstable air, yet, as a 

 whole, this phase of science seems to be somewhat back- 

 ward in the formulation of the ideas of processes, so that 

 their greater successful application is seen in geology. It 

 appears that the reciprocal dynamic relations which exist 

 between barometric lows and highs (both temporary and 

 permanent) makes them dynamic centers of action (Fas- 

 sig, '99) in a cycle of circulation and adjustment to stress. 

 This idea is one which may profitably be extended to the 

 interpretation of successive phases in the establishment 

 of climatic dominance. The change from the Ice Age to 

 that of the present, and the accompanying change of 

 storm tracks (Adams, '09, pp. 45-46) are comparable to 

 the seasonal change from winter to summer dominance 

 while passing through the transit i<)iinl Maivli w<>nther 

 stage. Furthermore, the suniiiuT and wiiit.T domiiiance 

 are dynamic equilibria established by a balancing among 



