648 GEOLOGY. 



been broken up by deformation^ or cut away by erosion. Hence there 

 was need for some reliable means of matching the beds of separated 

 series, and of maldng up a complete ideal series. This means is found 

 in the fossils they contain. While the variations of the faunas and 

 floras in different regions, and their migrations, introduce some minor 

 difhculties, the relations of the fossiliferous beds of one region to those 

 of another can be determined with great satisfaction, and often with 

 great precision. This is particularh^ so when abundant floating or free- 

 swimming species lived in the seas and were freely fossilized, for they 

 were deposited on the coasts of all the continents at practically the same 

 time, and no uncertainties from migration or local differences in rate 

 of evolution intervened to tlirow doubt upon the correlation. Without 

 the aid of fossils, the correlation of the deposits on the separate conti- 

 nents would be attended with grave obstacles and much uncertainty, 

 if not with quite prohibitive difficulties. 



B. Special Modes of Aggregation and of Movement. 



Inorganic solid matter is chiefly crystalloidal; organic matter is 

 chiefly colloidal; but there are colloidal states of inorganic matter and 

 there are cr^^stalloids among the organic products. In the inorganic 

 world, solids very generally tend to organize in the form of crystals; 

 in the organic world, they as generally tend to organize in the form of 

 cells. Neither tendency is complete or exclusi^'e, but each is dominant 

 in its own sphere. 



Still more distinctive than the formation of cells is the growth of 

 complex organized bodies, the differentiated members of which perform 

 special functions for one another, and are mutually dependent on one 

 another. This is a profound departure from the habitual modes of the 

 inorganic world. 



Still more so is the power of voluntary motion in more or less disre- 

 gard of outside physical influences. Through this power, distribution 

 may take place contrary to current and Avind, and to gravitation itself. 

 !Prom the view-point of past geologic transportation, this is perhaps 

 more singular than important, for no great mass of matter has been 

 transported contrary to the influences of gravity, wind, and current, 

 by the exercise of this peculiar power of animals, but it is not without 

 geologic importance in the migrations and in the redistributions of 

 organic influences that arise from migrations. When the influence of 



