554 POPULAR SCIENCE MONTHLY. 



Thenceforward two views of paleontology and two modes of 

 study gradually differentiated from one another, the one zoologi- 

 cal, the other geological. In the one case we study fossils in 

 taxonomic groups — i. e., as species, genera, families, orders, etc. — 

 and trace the gradual evolution of each of these from generalized 

 forms to their specialized outcomes, completing as far as possible 

 the genetic chain through all time. In the other we study fossils 

 in faunal groups, as successive geological faunas, and the geo- 

 graphic diversity in each geological period — i. e., the evolution of 

 geologic faunas and the causes of geographic diversity in each. In 

 a word, we study the laws of distribution of faunas in time geo- 

 logically 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 belongs 

 properly to geology. In this purely geologic paleontology, 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 brought 

 forward in these very last years of the century. 



I. GE^^ERAL LaW'S 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 be the same 

 everywdiere. A time horizon would be easily determined by iden- 

 tity 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 

 different conditions, evolution takes different directions and differ- 

 ent rates, producing geographical diversity in each geological pe- 

 riod. 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 approximately to determine any geological horizon. The 

 history of each country must be studied for itself. A general his- 

 tory 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, 



