This, of course, is pure conjecture. All that can be said of it at present is that it is not opposed to observed facts. That mutations do occur must be admitted. At present we are totally in the dark as to what causes them. They arise at the most unexpected times.
In favour of the explanation based on “mutation” there is the interesting fact that geographical isolation does not by any means always cause divergence of character. This Romanes, with great fairness, freely admits. “There are,” he writes, on p. 133 of vol. iii. of Darwin and after Darwin, “four species of butterflies, belonging to three genera (Lycæna donzelii, L. pheretes, Argynnis pales, Erebia manto), which are identical in the polar regions and the Alps, notwithstanding that the sparse Alpine populations have been presumably separated from their parent stocks since the glacial period.” Again, there are “certain species of fresh-water crustaceans (Apus), the representatives of which are compelled habitually to form small isolated colonies in widely separated ponds, and nevertheless exhibit no divergence of character, although apogamy has probably lasted for centuries.”
Cormorants
To these examples we may add that of the cormorants. These birds have an almost worldwide range. One species—our Cormorant (Phalacrocorax carbo)—occurs in every imaginable kind of environment. Isolation has not effected any changes in the appearance of this species. Yet in New Zealand there exist no fewer than fourteen other species of cormorant. New Zealand is a country where climatic conditions are comparatively uniform, nevertheless it boasts of no fewer than fifteen out of the thirty-seven known species of cormorant. A possible explanation of this phenomenon may be found in the comparatively easy conditions under which cormorants live in New Zealand.[10] Under such circumstances mutants may be permitted by natural selection to survive, whereas in other parts of the world such mutants have not been able to hold their own.
Prof. Bateson has likened natural selection to a competitive examination to which every organism must submit. The penalty for failure is immediate death. The standard of the examination may vary with the locality.
Isolation, then, is a very important factor in the making of species, for without it, in some form, the multiplication of species is impossible.
Let us, in conclusion, briefly summarise what we now know of the method in which new species are made. We have studied the various factors of evolution—variation and correlation, heredity, natural selection, sexual selection, and the other kinds of isolation. How do these combine to bring new species into being, and to establish the same?
Natural Selection
Let us first consider the factor known as natural selection, since this is the one on which Darwin laid such great stress. Natural selection, although a most important factor in evolution, is not an indispensable one. Evolution is possible without natural selection.
Let us suppose that there is no such thing as natural selection; that the numbers of existing species are kept constant by the elimination of all individuals born in excess of the number required to maintain the species at the existing figure, and that the elimination of the surplus is effected, not by natural selection, but by chance, by the drawing of lots. Under such circumstances there may be evolution, existing species may undergo change, but the evolution will be determined solely by the lines along which variations occur.