Seite - 502 - in The Origin of Species

502 ORIGIN OF SPECIES cated varieties when crossed and their mongrel offspring are perfectly fertile. Turning to geographical distribution, the difficulties en- countered on the theory of descent with modification are serious enough. All the individuals of the same species, and all the species of the same genus, or even higher group, are descended from common parents ; and therefore, in however distant and isolated parts of the world they may now be found, they must in the course of successive generations have travelled from some one point to all the others. We are often wholly unable even to conjecture how this could have been effected. Yet, as we have reason to believe that some species have retained the same specific form for very long periods of time, immensely long as measured by years, too much stress ought not to be laid on the occasional wide dif- fusion of the same species; for during very long periods there will always have been a good chance for wide migra- tion by many means. 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 as to the full extent of the various climatal and geographical changes which have affected the earth dur- ing modern periods; and such changes will often have facili- tated migration. As an example, I have attempted to show how potent has been the influence of the Glacial period on the distribution of the same and of allied species throughout the world. We are as yet profoundly ignorant of the many occasional means of transport. With respect to distinct species of the same genus inhabiting distant and isolated regions, as the process of modification has necessarily been slow, all the means of migration will have been possible dur- ing a very long period ; and consequently the difficulty of the wide diffusion of the species of the same genus is in some degree lessened. As according to the theory of natural selection an inter- minable number of intermediate forms must have existed, linking together all the species in each group by gradations as fine as are our existing varieties, it may be asked,Why do we not see these linking forms all around us ? Why are not all organic beings blended together in an inextricable chaos?