5O GENERAL PRINCIPLES OF ZOOLOGY. 



therefore have been finally determined by the favorable- 

 ness or the unfavorableness of the conditions of life pre- 

 vailing in the various regions, as the climate, food-supply, 

 etc. If on the other hand, we assume that the animal 

 species have arisen from one another through variation, 

 then there must have been, as an influence determining the 

 manner of distribution, besides the conditions of existence, 

 still a second factor, which we will call the geological. We 

 know that the configuration of the earth's surface has in 

 many respects changed in the course of the enormous 

 space of time of the geological periods ; that land areas, 

 which earlier were united, have become separated by the 

 encroachments of the sea; that by the upheaval of moun- 

 tains also important barriers to the distribution of animals 

 w r ere formed. From the circumstance that these two 

 changes, the changes in the earth's surface and in the 

 animal world established upon it have gone on hand in 

 hand there follows necessarily the consequence that so 

 much greater differences in the faunal character of two 

 lands must result the longer they have developed inde- 

 pendently of one another, without interchange of their 

 animal populations, and the longer the inhabitants have 

 been separated by impassable barriers. For the various 

 animal groups the character of the barriers is different ; 

 terrestrial animals which cannot fly are hindered in their 

 distribution by arms of the sea ; sea-dwellers contrariwise by 

 land areas ; for terrestrial mollusks high mountain ranges, 

 which are dry and barren, or constantly snow-capped, are 

 effectual. 



Instances of Proofs. Since attention has been given 

 to these conditions, many geographical facts favorable to 

 the descent theory have been ascertained: (i) Of the 

 various continents Australia has faunally an independent 

 character; when discovered it contained almost none of 

 the higher (placental) mammals, except such as can fly 

 (Chiroptera), or marine forms (Cetacea), or such as are 

 easily transported by floating wood (small rodents), or such 

 as were introduced by man (Dingo, the Australian dog); 



