In his own words:
Turning to geographical distribution, the difficulties encountered on the theory of descent with modification are grave enough.
All the individuals of the same species, and all the species of the same genus, or even higher group, must have descended from common parents; and therefore, in however distant and isolated parts of the world they are now found, they must in the course of successive generations have passed from some one part to the others.
We are often wholly unable even to conjecture how this could have been effected…A broken or interrupted range may often be accounted for by the extinction of the species in the intermediate regions.
It cannot be denied that we are as yet very ignorant of the full extent of the various climatal and geographical changes which have affected the earth during modern periods; and such changes will obviously have greatly facilitated migration.
Photo credit: biogeographic distribution from http://ib.berkeley.edu/labs/mcguire/Sulawesidracos.72.jpg
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