What is Darwin’s point in chapter 11?
In chapter 11, Darwin discusses the patterns of distribution of life on the earth. This chapter again seems to address more the prevalent “theory of creation” (where species are fixed and placed at specific points) rather than give substantive support for natural selection.
“These cases of relationship, without identity, of the inhabitants of seas now disjoined, and likewise of the past and present inhabitants of the temperate lands of North America and Europe, are inexplicable on the theory of creation. We cannot say that they have been created alike, in correspondence with the nearly similar physical conditions of the areas; for if we compare, for instance, certain parts of South America with the southern continents of the Old World, we see countries closely corresponding in all their physical conditions, but with their inhabitants utterly dissimilar.”
Specifically, he attacks the notion that the same species were created independently at separate points of the globe. And he believes instead that species arose at a single point and dispersed. On this point I would actually agree with him and see no incompatibility with the biblical model of creation.
“We are thus brought to the question which has been largely discussed by naturalists, namely, whether species have been created at one or more points of the earth’s surface. Undoubtedly there are very many cases of extreme difficulty, in understanding how the same species could possibly have migrated from some one point to the several distant and isolated points, where now found. Nevertheless the simplicity of the view that each species was first produced within a single region captivates the mind.”
“Hence it seems to me, as it has to many other naturalists, that the view of each species having been produced in one area alone, and having subsequently migrated from that area as far as its powers of migration and subsistence under past and present conditions permitted, is the most probable.”
But even though a species arose at a single location, it does not show that the species arose from another species. It could still have been created at that single location and then dispersed through the mechanisms that Darwin elaborates in the chapter.
In regards to how life disperses, Darwin shows that the main factor is physical barriers, rather than climate or other physical conditions.
“our general review is, that barriers of any kind, or obstacles to free migration, are related in a close and important manner to the differences between the productions of various regions.”
“neither the similarity nor the dissimilarity of the inhabitants of various regions can be accounted for by their climatal and other physical conditions.”
“No two marine faunas are more distinct, with hardly a fish, shell, or crab in common, than those of the eastern and western shores of South and Central America; yet these great faunas are separated only by the narrow, but impassable, isthmus of panama. ”
Darwin also presents some possible theories on how life disperses, specifically with plants. Some he mentions are drift wood, beaks and feet of birds, crops of birds, and icebergs. And the final part of the chapter he discusses the effects of the glacial period.
But, he also acknowledges that he cannot fully account for how natural selection can explain the dispersion of life. He notes two examples which I find interesting.
“it is certainly a wonderful fact that New Zealand should have a closer resemblance in its crustacea to Great Britain, its antipode, than to any other part of the world.”
“twenty-five species of Algae are common to New Zealand and to Europe, but have not been found in the intermediate tropical seas.”
And at the end, he states:
“I am far from supposing that all difficulties are removed on the view here given in regard to the range and affinities of the allied species which live in the northern and southern temperate zones and on the mountains of the intertropical regions. Very many difficulties remain to be solved. I do not pretend to indicate the exact lines and means of migration, or the reason why certain species and not others have migrated; why certain species have been modified and have given rise to new groups of forms, and others have remained unaltered. We cannot hope to explain such facts, until we can say why one species and not another becomes naturalised by man’s agency in a foreign land; why one ranges twice or thrice as far, and is twice or thrice as common, as another species within their own homes.”
Again, overall, he doesn’t provide any evidence for natural selection in this chapter, but rather simply attacks an erroneous view of creation that was then held by many.