GEOGRAPHICAL EVIDENCE 385 



Geographical evidence. It has been found that every group of 

 organisms expands its range just as far as conditions permit. Re- 

 gions in every way similar, so far .as climate, soil, and other condi- 

 tions are concerned, are inhabited by totally different plants and 

 animals. Thus, the climate of Australia is not very different 

 from some of North America, but the animals and plants living 

 there naturally are not like those of North America. The same is 

 true about other similar regions. 



Regions that are very different are occupied by forms of plants 

 and animals that are sufficiently similar to be considered of the 

 same families. For example, goats and sheep, obviously related to 

 each other, are found in tropical, temperate, arctic, and antarctic 

 circles. They are thus living in varied surroundings. 



Darwin pointed out that where similar regions are occupied by 

 different flora and fauna, these regions were always separated from 

 each other by impassable barriers such as oceans, mountain 

 ranges, and deserts. Thus Australia and America were always 

 separated by the ocean, and land animals could not migrate from 

 Australia to America. On the other hand, where similar plants 

 and animals inhabit regions that are markedly different in their 

 climate and soil, these regions are either connected directly, or 

 show evidence of having been connected in the past. For ex- 

 ample, the plants and animals found in oceanic islands are fre- 

 quently related to the inhabitants of the nearest mainland. 

 There is some evidence that tends to show that the islands were 

 a part of the mainland at one time. 



THE MONGOLIAN EXPEDITIONS. It has been known for some 

 time that Behring Strait formerly existed as a land bridge con- 

 necting America with Asia. It is thought that a similar connec- 

 tion, by way of Greenland, connected America with Europe. It 

 was observed that the animals of North America, Europe, and 

 Asia north of the Himalaya Mountains were somewhat similar 

 and it was thought that animals must have crossed these areas 



