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University of California. 



[Vol, i. 



steady advance of the whole organic kingdom, everywhere and all 

 along the line, under the operation of all the factors of evolution 

 known and unknown, whatever these may be. If this were all, there 

 would be successive geological ', but no simultaneous geographical , 

 faunas and floras. Organic forms would change in time but not dif- 

 ferentiate in space. The determination of synchronism or of a geo- 

 logical horizon in different continents, would be easy because the 

 identity of fossil forms of the same time in all parts of the earth would 

 be absolute. A chronology of the earth would be easy to construct, 

 because it would be the same everywhere. There would be every- 

 where but reprints of the same history. This is one extreme, but 

 'we know it is far from the real truth. 



Origin and Increase of Geographical Diversity. — Different condi- 

 tions in different places isolated from one another by barriers, phys- 

 ical, climatic, or both, and therefore without mixture of species by 

 migration, give rise to different rates or directions of advance and 

 therefore to divergence of geographical faunas and floras, which in- 

 crease without limit with time. If this were all, there would long 

 ere this have come about an extreme diversity of geographical faunas 

 and floras; more extreme than any we now know. Indeed, so ex- 

 treme that determination of synchronism of strata of different places 

 by comparison of their fossils would be impossible. The geological 

 history of each country must be studied for itself. There could be 

 no general history of the earth and no general classification of the 

 strata. This is the other extreme; but neither is this true. 



Critical Periods Diminish Geographical Diversities. — But from 

 time to time, at great intervals, there occur critical periods, i. c, 

 periods of great and widespread changes in physical geography, 

 and therefore of climate, and therefore, also, in organic forms. These 

 latter changes, as already explained, are determined partly by the 

 pressure of a changed environment and partly by wide migrations 

 — both permitted by removal of barriers and enforced by changes of 

 climate — and the consequent mingling of faunas and floras previ- 

 ously separated, the severer struggle consequent thereon and more 

 rapid rate of evolutionary changes. In connection with the new 

 conditions, the severer struggle and more rapid evolution, there 

 would arise at these times new and higher dominant types which 



