When the Origin was published, the idea that species were not fixed entities had been in the air for some time, thanks to Lamarck, Robert Chambers, anonymous author of the best-selling Vestiges of the Natural History of Creation, and Darwin's own grandfather, Erasmus.
But unlike those men, Darwin put all the different pieces together into a coherent whole. How was that? Chapter 11 of the Origin, 'Geographical distribution', gives some hints.
First there's the nurture. In the age of long-haul flights and wildlife documentaries, it's easy to forget how difficult it was to see different plants and animals even a few decades ago. To get an idea of how the world changes, and what nature is capable of, you really need to go to the tropics. In Darwin's day that involved spending several years on a boat; Darwin was lucky enough to get that opportunity.
But plenty of educated young men could have sailed on the Beagle, come back with a tidy haul of specimens, and been none the wiser how they got that way. Beneath the idle youth, country-squire lifestyle, pigeon fancying and gut ache, Darwin also had the good fortune to be a genius.
In particular, I think, he had a genius for looking at the living world and seeing how variation in space reflected processes in time. This is a theme that recurs through the book: in the previous chapter, he used the differences between the snails and beetles of Madeira and Europe to infer that the island had seen more recent evolutionary change than the mainland.
In this chapter, those space/time insights take centre stage. He shows that discontinuity in space, such as the differences between the fauna of South America and Australia, despite their similar environments, is explained by continuity in time -- the species living there now are descended from the ones that lived there before.
"We see in these facts some deep organic bond, prevailing throughout space and time, over the same areas of land and water, and independent of their physical conditions. The naturalist must feel little curiosity, who is not led to inquire what this bond is.
This bond, on my theory, is simply inheritance, that cause which alone, as far as we positively know, produces organisms quite like, or, as we see in the case of varieties nearly like each other."
This answers, says Darwin "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". No, they haven't: "[I]f the same species can be produced at two separate points, why do we not find a single mammal common to Europe and Australia or South America?"
This doesn't mean that the same thing can't show up in different places. Immediately after that sentence comes this:
"The conditions of life are nearly the same, so that a multitude of European animals and plants have become naturalised in America and Australia; and some of the aboriginal plants are identically the same at these distant points of the northern and southern hemispheres? The answer, as I believe, is, that mammals have not been able to migrate, whereas some plants, from their varied means of dispersal, have migrated across the vast and broken interspace."
Throughout the Origin, Darwin often deals with opposing concepts: variation and inheritance, for example. But he seldom lets opposing ideas collide; a shame, because I often wish he would say something like: 'So if this happens, and this happens, what should we expect the result to be?' Instead, he tends to set them up in parallel, acknowledging that both are forces in the world, but leaving us to conclude that whatever we see must result from their combined action.
To address the question of dispersal, Darwin turns to another side of his intellect: the experimentalist. The passages where he describes his experiments (on Instinct, for example) are some of the most fun to read in the book. Here he writes about a series of what must have been quite smelly investigations.
He studied, for example, the capacity of seeds and plants to germinate after a month or few floating in salt water:
"To my surprise I found that out of 87 kinds, 64 germinated after an immersion of 28 days, and a few survived an immersion of 137 days."
He also experimented with the buoyancy of fresh and dried seeds, fruits and stones, seeds attached to branches, and so on. A back-of-the-envelope calculation gives a rough idea of dispersal potential:
"In Johnston's physical Atlas, the average rate of the several Atlantic currents is 33 miles per diem (some currents running at the rate of 60 miles per diem); on this average, the seeds of 14/100 plants belonging to one country might be floated across 924 miles of sea to another country; and when stranded, if blown to a favourable spot by an inland gale, they would germinate."
But seeds have more ways of getting around than just floating.
"[P]eas and vetches, for instance, are killed by even a few days' immersion in sea-water; but some taken out of the crop of a pigeon, which had floated on artificial salt-water for 30 days, to my surprise nearly all germinated."
"In the course of two months, I picked up in my garden 12 kinds of seeds, out of the excrement of small birds, and these seemed perfect, and some of them, which I tried, germinated."
"Some hawks and owls bolt their prey whole, and after an interval of from twelve to twenty hours, disgorge pellets, which, as I know from experiments made in the Zoological Gardens, include seeds capable of germination."
"I forced many kinds of seeds into the stomachs of dead fish, and then gave their bodies to fishing-eagles, storks, and pelicans [where did he find those?]; these birds after an interval of many hours, either rejected the seeds in pellets or passed them in their excrement; and several of these seeds retained their power of germination."
In the way it brings together Darwin the explorer and observer, Darwin the experimenter and Darwin the theorist, this chapter contains some of the most thoroughly convincing parts of the entire book.
Recently, DNA sequencing has shown that history does not always trump environment. Matt Lavin and his colleagues, for example, have found, contra Darwin, that leguminous plants living in Africa are more closely related to those living in the same environments in South America than they are to African legumes in different environments. To quote Ran Nathan, in recent years, it's come to look as if intercontinental dispersal, although rare, 'is of critical importance for natural populations and communities'.
Darwin didn't have all the answers. He knew that continents rise and fall, but he didn't know they also drift from side to side. What he did know about were ice ages, proposed by Louis Agassiz in 1837. In the absence of plate tectonics, climate change acts as a kind of universal solvent, shunting species to wherever Darwin needs them to go.
For example, you find similar species on the mountain tops of the Alps and Pyrenees -- apparently one of the main pieces of evidence for multiple centres of creation -- because they got there when the climate was arctic, and were stranded when the ice retreated.
On the other hand, he suggests, during warmer periods, species might have extended north into the Arctic, where the gaps between continents narrow, and so spread between Eurasia and America.
And during ice ages, temperate species would have been pushed so close to the equator that it wouldn't have been a problem for them to cross hemispheres, and carry on moving as the climate warmed.
"Thus, as I believe, a considerable number of plants, a few terrestrial animals, and some marine productions, migrated during the Glacial period from the northern and southern temperate zones into the intertropical regions, and some even crossed the equator. As the warmth returned, these temperate forms would naturally ascend the higher mountains, being exterminated on the lowlands; those which had not reached the equator, would re-migrate northward or southward towards their former homes; but the forms, chiefly northern, which had crossed the equator, would travel still further from their homes into the more temperate latitudes of the opposite hemisphere."
I think Darwin might be overestimating the potential of glaciation here - as far as I know, ice ages don't move temperate species to the equator, shuffle them, and then cut the deck in a new combination. Although if anyone has heard different, please tell me.
Hey, not even geniuses are omniscient.
Next: Geographical distribution, continued. Let's hope I've kept some powder dry.
Just a clarification question. So Darwin is saying that living things did not separately start on different continents. Life started at one location, then evolved into plants, which stayed relatively in the same location, then evolve into animals which could carry the plants to different locations? Did plants evolve before animals?
This is just a question. How do the species migrate from place to place. For example, before there were boats and planes to bring new species to foreign lands, how did the land mammal get from continent to continent?