A CENTUEY OF GEOLOGY. 285 



period, i. c, the evolution of geologic faunas and the causes of 

 geographic diversity in each. In a word, we studj- the laws of distribu- 

 tion of faunas in time geologically and in space geographically, and 

 the causes of these laws in each case. The first is strictly a branch of 

 zoology and botany, and we leave it to these specialists. The second 

 alone l)elongs properly to geology. In this purely geologic paleon- 

 tology, as seen from its scope given above, there are many questions 

 of widest philosophical interest which are only now attracting the 

 attention they deserve. I only touch lightly two which have been 

 ])rought forward in these very last years of the century. 



I. General Jcnvs of faunal evolution. — The evolution of the organic 

 kingdom from this strictly geological point of view may be briefly 

 formulated as follows: 



1. Throughout all geological time there has been a general movement 

 upward and onward, as it were abreast, everywhere. If this were all, 

 there would be only geological progress, but no geographical diversity. 

 Geological history would l)e the same everywhere. A time horizon 

 would be easily determined b}' identity of fossil species. This we 

 know is not true. Therefore there are other elements besides this. 



2. In different countries, isolated from one another and under 

 difi:erent conditions, evolution takes different directions and dift'erent 

 rates, producing geographical diversitv in each geological period. 

 This diversity increases with time as long as the isolation continues. 

 If this were all, the geographical diversity by continued divergence 

 would have become so great that it would be impossible even approxi- 

 matel}' to determine any geological horizon. The histor}^ of each 

 country uuist be studied for itself. A general histor}' of the earth 

 would be impossible. But this also is not true. There is, therefore, 

 still another element. 



3. From time to time, at long intervals, i. e. , critical periods, there are 

 widespread readjustments of the crust to internal strain, determining 

 changes of physical geography and of climate, and therefore wide 

 migrations of species, with mingling and conflict of faunas. This 

 would produce more rapid movement of evolution, but at the same 

 time more or less complete obliteration of geographical diversity. 



4. After these periods of migrations and minglings there would be 

 reisolations in new localities, and the process of diversification would 

 recommence and increase as long as the isolation continues. 



The last of these critical periods of migrations and minglings and 

 struggles for life among competing species was the Glacial epoch, or 

 ice age. Therefore the present geographical distribution of species 

 was largely determined by the extensive migrations of that time. 



II. Cosmopolitan and provincial faunas.— Thexf' are apparently in 

 the history of the earth periods of widespread or cosmopolitan faunas, 

 alternating with localized or provincial faunas. The cosmopolitan 



