I’m still reading The Origin of Species and found another passage that made me think of Jared Diamond’s Guns, Germs, and Steel.
It is a remarkable fact, strongly insisted on by Hooker in regard to America, and by Alph. de Candolle in regard to Australia, that many more identical plants and allied forms have apparently migrated from the north to the south, than in a reversed direction. . . . I suspect that this preponderant migration from north to south is due to the greater extent of land in the north, and to northern forms having existed in their own homes in greater numbers, and having consequently been advanced through natural selection and competition to a higher stage of perfection or dominating power, than the southern forms. And thus, when they became commingled . . . the northern forms were enabled to beat the less powerful southern forms. Just in the same manner as we see at the present day, that very many European productions cover the ground in La Plata, and in a lesser degree in Australia, and have to a certain extent beaten the natives; whereas extremely few southern forms have become naturalised in any part of Europe. . . (The Origin of Species, pp. 370-71 in the Penguin Classics edition).
What Darwin observes in the plant kingdom — namely, that it is generally the northern (and specifically Eurasian) forms that have successfully invaded the south, rather than vice versa — has its parallel within the human species. It is disproportionately those races that developed on the great northern continent of Eurasia that have been successful in invading other continents and displacing other peoples, a phenomenon which Diamond attempts to explain in his book Guns, Germs, and Steel.
Diamond, an enlightened anti-racist, casts his explanation mostly in terms of culture and technology rather than biological evolution, invoking genes only to explain racial differences in resistance to particular diseases, and would of course never dream of using Darwin’s language about advancing “to a higher stage of perfection” — but for all that, his ultimate explanation is essentially the same as Darwin’s: that Eurasia is simply bigger. Diamond also notes that, in addition to being larger in absolute terms, Eurasia has the further advantage of being oriented east-to-west, which means that any given climatic zone on the Eurasian continent is likely to be wider than the corresponding zones on north-to-south continents such as Africa and the Americas. For Diamond, Eurasia’s size and orientation facilitates the wide dispersion of domestic animals and technological advances. Based on Darwin, we can add that a larger population and easy migration between regions would mean more mutations, more intense competition, and therefore accelerated evolution.