CHAPTER 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 
