HISTORY OF ZOOLOGY 33 



Distribution of Animals. — From Animal Geography we learn that 

 the present distribution of animals is the product of the past. It will 

 therefore be possible from this to figure out many of the earlier conditions 

 of things. 



If we assume that from the beginning all animal species were consti- 

 tuted as they now are, they would then have been placed by the purposeful 

 Creator in the regions best suited to them; their distribution would there- 

 fore have been determined by favorable or unfavorable conditions of 

 life prevailing 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 iniluence 

 determining the manner of distribution, besides the conditions of exis- 

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

 that the configuration of the earth's surface has changed in many respects 

 in the course of the enormous time of the geological periods; that land 

 areas which earlier were united, have become separated by the encroach- 

 ments of the sea; that by the upheaval of mountains important barriers 

 to the distribution of animals were also formed. On the other hand 

 regions which were formerly separated have become connected; islands, for 

 example, being united by the emergence of land from the sea. From the 

 fact 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 consecjuence that greater differences in the faunal 

 character of two lands must result, the longer the inhabitants have been 

 separated by impassable barriers. For the various groups the character 

 of the barriers is different; terrestrial animals, which cannot fly, are 

 hindered in their distribution by the sea; marine forms by land barriers; 

 for terrestrial molluscs mountain ranges, which are dry and barren, or con- 

 stantly snow-capped, are effectual. 



Instances of Proofs. — Since attention has been called to these 

 conditions, many facts favorable to the theory of evolution have been 

 ascertained: (i) Of the various continents Australia has faunally an in- 

 dependent 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 could be introduced by man (dingo, the 

 AustraHan dog); instead, it had remarkable birdlike animals (with beak 

 and cloaca), and the marsupials, which have become extinct in the Old 

 World and, opossums excepted, in America as well. The phenomenon 

 is explained by the geological fact that in the earth's history x-lustralia, 

 with its surrounding islands, was certainly the earliest to lose its connec- 



