222 HABIT AND INTELLIGENCE. [CHAP. 



not only true, but true without any other modifying cause 

 — in other words, if organization depended on the con- 

 ditions and mode of life alone — it would be almost a 

 necessary inference, that if in any two regions the 

 conditions of life as to climate, food, and other external 

 circumstances are nearly similar, the organic populations 

 of the two ought also to be nearly similar ; and the degree 

 External of similarity between the organic populations of any two 

 stances do regions ought to be in some sort of proportion to the 

 not deter- similarity between the conditions of life in the two. But 



7iiine dis- 1 . 1 1 . . . 



tribution. there IS no such relation — not even an approximation to it. 

 The degree of resemblance between the organic populations 

 of any two regions depends in scarcely any degree on the 

 similarity of climate and other conditions, but almost 

 entirely on their contiguity or distance, and the facility 

 with which they may have been colonised the one from 

 the other, or both from the same source. For instance. 

 Mountain mountain regions, when distant from each other, though 

 species. having similar climates, are not found to have similar 

 organic populations : on the contrary, the peculiar species 

 of each mountain region are, for the most part, nearly 

 related to the species inhabiting the neighbouring low 

 country, but modified, as if to suit the mountain life. The 

 inference is obvious, and I believe true, that the mountain 

 species are, not metaphorically, but literally kindred with 

 those of the plain : either the one group of species has 

 been descended from the other, or both have been descended 

 from the same source. I will mention another special and 

 very remarkable instance of the way in which species are 

 distributed. The three regions of Australia, South Africa, 

 and the southern part of South America are situated 

 between nearly the same parallels, and have nearly the 

 same climate (including, of course, in the word climate, 

 moisture as well as temperature) ; and climate is much 

 more important in determining the conditions of life than 

 all other influences put together. On the other hand, 

 the three old continents, Europe, Asia, and Africa, contain 

 every variety of climate, from the coldest to the hottest, 

 from the driest to the wettest. But Australia, South Africa 



