Lb Conte.1 



Critical Periods. 



317 



of the earth as a whole, whether by secular cooling and contraction 

 or by slowing down of rotation by tidal friction or otherwise, there 

 must have been, from time to time, readjustments of the crust affect- 

 ing the whole earth. 



CRITICAL PERIODS. 



The thesis which I wish to maintain, then, is, that there have 

 been what I have elsewhere called "critical periods" in the history 

 of the earth,* i. e., periods of very general readjustments of the 

 crust of the earth, and therefore of widespread changes in physical 

 geography, so great and so general as to affect profoundly and 

 widely the climates of the earth ; that these physical changes in 

 their turn gave rise to still more marked changes in organic forms ; 

 and finally that all these changes together form a rational basis for 

 the primary divisions of time. 



These great and comparatively rapid changes in organic forms 

 are produced in the following ways: I. The changes in physical 

 geography open gateways and permit migrations in many directions. 

 2. The changes in climate, besides their direct effect on organism, 

 compel migrations mainly north and south. 3. These migrations in 

 their turn precipitate different faunas and floras upon one another, 

 producing severe struggles between invaders and natives, and there- 

 fore the destruction of many forms of both, and large modification 

 of the survivors. 4. The foreign invasion compels many natives in 

 their turn to migrate, and so the wave of invasion, of severer strugf- 

 gle, and of consequent changes is propagated as far as physical con- 

 ditions will allow migration. The effect of all this must be a more 

 rapid rate of evolution of organic forms, as the result (a) of a new 

 environment, and (b) of a severer struggle for life. The more rapid 

 rate of evolution, and especially new opportunities, give rise to higher 

 dominant classes. These higher dominant classes again in their 

 turn determine changes in lower forms, especially their immediate 

 rivals, and these changes are again propagated downward through 

 the whole organic kingdom and compel a new adjustment of the 

 whole on a different basis. 



The great theater of physical changes, of extensive migrations, 



*Am. jour., vol. 14, p. 22, 1877. 



