CHAPTEE XII 



THE GEOGRAPHICAL DISTRIBUTION OF ORGANISMS 



The facts to be explained — The conditions which have determined dis- 

 tribution — The permanence of oceans — Oceanic and continental areas 

 — Madagascar and New Zealand — The teachings of the thousand- 

 fathom line — The distribution of marsupials — The distribution of 

 tapirs — Powers of dispersal as illustrated by insular organisms — Birds 

 and insects at sea — Insects at great altitudes — The dispersal of plants 

 — Dispersal of seeds by the wind — Mineral matter carried by the wind 

 — Objections to the theory of wind-dispersal answered — Explanation 

 of north temperate plants in the southern hemisphere — No proof of 

 glaciation in the tropics — Lower temperature not needed to explain 

 the facts — Concluding remarks. 



The theory which we may now take as established — that all 

 the existing forms of life have been derived from other forms 

 by a natural process of descent with modification, and that 

 this same process has been in action during past geological 

 time — should enable us to give a rational account not only of 

 the peculiarities of form and structure presented by animals 

 and plants, but also of their grouping together in certain 

 areas, and their general distribution over the earth's surface. 



In the absence of any exact knowledge of the facts of 

 distribution, a student of the theory of evolution might 

 naturally anticipate that all groups of allied organisms would 

 be found in the same region, and that, as he travelled farther 

 and farther from any given centre, the forms of life would 

 differ more and more from those which prevailed at the 

 starting-point, till, in the remotest regions to which he could 

 penetrate, he would find an entirely new assemblage of 

 animals and plants, altogether unlike those with which he was 



