EVOLUTION 469 



492. Geographic evidence. We expect every group of organisms 

 to expand its range just as far as conditions permit, and we rather 

 expect a given kind of situation to maintain one kind of population 

 and a different kind of situation to maintain a different kind of popu- 

 lation. Yet when we examine the distribution of species over the 

 surface of the earth, certain curious facts appear. 



In the first place, we find regions in every way similar, so far as 

 climate, soil, etc. are concerned, inhabited by totally different plants 

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

 that of most of Europe and large parts of Africa, Asia, North America, 

 and South America ; but when Europeans first came there, they found 

 plants and animals that are not found living naturally in other parts 

 of the world. The same kinds of facts are found in abundance on 

 comparing many regions with one another. 



In the second place, we find regions that are very different occupied 

 by forms of plants and animals that are sufficiently similar to be con- 

 sidered as of the same families. Thus, goats and sheep, obviously 

 related to each other genetically, are found in the tropical zones as well 

 as in the temperate, and well up to the arctic and antarctic circles, 

 living in many varied kinds of surroundings. 1 



Darwin pointed out that where we have similar regions occupied 

 by different flora and fauna, these regions are always separated 

 from each other by impassable barriers, as oceans, mountain ranges, 

 deserts, etc. On the other hand, where we find similar plants and 

 animals inhabiting regions that are markedly different in their climate, 

 soil, etc., these regions are connected directly or show evidence of 

 having been connected in the past. For example, the plants and 

 animals found in oceanic islands are frequently quite distinct from 

 those found elsewhere, but they are also as a rule closely related to 

 the inhabitants of the nearest mainland. Facts of this kind can easily 

 be explained by the assumption that all the organisms are derived 

 from ancient forms, with modifications, and they cannot be easily 

 explained in any other way. 



493. Summary. The evidences for organic evolution or for 

 the descent of plants and animals from common ancestors, with 

 divergence from ancient types, are (i) palaeontological, the 



1 Of course we are now speaking of the natural range of various wild 

 species. 



