3i8 



University of California. 



[Vol. i. 



and of severe struggle, and therefore of rapid evolution, especially 

 of higher forms, and therefore also the place of first appearance of 

 dominant classes has undoubtedly been what Huxley calls Arctogcea, 

 i. e., North America, and Eurasia north of the Himalayas and Sahara, 

 or all the northern hemisphere north of Central America, Sahara, 

 and the Himalayas. This, the greatest body of contiguous or con- 

 tinuous land, has in later geological times been sometimes united, 

 sometimes divided. It has been subject to the greatest changes, the 

 widest migrations, the severest conflicts, and therefore the most rapid 

 evolution of dominant forms. But these dominant forms have from 

 time to time, as opportunity offered, invaded more southern lands, 

 and always as conquerors. 



Signs of Critical Periods. — These greatest revolutions are marked 

 in the rocks (i) by widespread unconformities, and therefore by a 

 loss of record which is greater in proportion as the unconformity is 

 more general. But since unconformity always means eroded land 

 surface, widespread unconformity must mean greatly enlarged con- 

 tinents, or a continental period between two comparatively oceanic 

 periods. (2) They are marked by great, very general, and appar- 

 ently sudden changes in organic forms, affecting not only species, 

 but also genera, families, and orders. (3) By the introduction of 

 new and higher dominant classes. (4) By the birth of great moun- 

 tain ranges. Of these characteristic signs the most general and 

 important is the change in organic forms. This is, therefore, the 

 most important means of determining the primary divisions of time, 

 but the other signs are usually concurrent and are therefore con- 

 firmatory. The secondary divisions are based on less general 

 changes, and are therefore themselves more local in their applica- 

 tion. 



It must not be imagined, however, that these great revolutions 

 of the earth's crust are catastrophic, in the sense of being instanta- 

 neous. On the contrary, although they were periods of exceptional 

 commotion, they continued probably hundreds of thousands of 

 years. Nor were they simultaneous everywhere in any mathemati- 

 cal sense. On the contrary, the changes were doubtless propagated 

 from place to place until readjustment was complete. So, also, and 

 still more certainly, the changes in organic forms were propagated 



