Chapter 12: Geographical Distribution, continued

Back when I started this, I remarked that one of the reasons I hadn't read the Origin was that I couldn't imagine it being essential to a grasp of contemporary science. Regarding evolution, I think you could still make a case for this. But in other ways, that statement shows that you really shouldn't opine on topics you know nothing about.

Specifically, I'm talking about ecology (by which, just to be clear, I mean the study of the interactions of living things with each other and their environment, rather than 'nature' or 'environmentalism'). It's been said that all European philosophy is a series of footnotes to Plato. Well, all ecology is a series of footnotes to Darwin.

The word itself hadn't been invented -- Ernst Haeckel did that in 1873. But throughout the book Darwin talks about the 'economy of nature', which amounts to the same thing. And chapter 12, 'Geographical Distribution, continued', made me think that, although we now have more data and have learnt to express things mathematically, ecology's stock of ideas hasn't expanded since Darwin's day.

One reason for this is that Darwin was head and shoulders above anyone who's studied ecology since. Another is that the techniques he used -- observing and measuring what plants and animals do in the field and the lab -- haven't changed that much in the subsequent 150 years, and still form the basis for a huge amount of ecological work.


The key questions that Darwin sets out to address in this chapter are no longer live issues. We can answer whether species have single or multiple areas of creation, and we know that when Darwin considered how plants and animals got around the world, he was right to say: "I cannot honestly admit Forbes's view on continental extensions, which, if legitimately followed out, would lead to the belief that within the recent period all existing islands have been nearly or quite joined to some continent."

Which makes it more remarkable that so much of this chapter still seems so fresh. As in the preceding chapter, Darwin grapples with the issue of how dispersal and evolution together create the fabric of nature. In the process he invents another subdiscipline: island biogeography.

Before that, though, he sets himself the task of explaining another hard case: why many freshwater species are distributed so widely, despite the patchiness of their habitat. His answer is that the capacity for the occasional long migration is an extension of "their having become fitted, in a manner highly useful to them, for short and frequent migrations from pond to pond, or from stream to stream".

Again, he makes his case through experiment. Chapter 11 was a hymn to the dispersing power of birds' intestines; here, he focuses on their feet:

"I suspended a duck's feet, which might represent those of a bird sleeping in a natural pond, in an aquarium, where many ova of fresh-water shells were hatching; and I found that numbers of the extremely minute and just hatched shells crawled on the feet, and clung to them so firmly that when taken out of the water they could not be jarred off, though at a somewhat more advanced age they would voluntarily drop off. These just hatched molluscs, though aquatic in their nature, survived on the duck's feet, in damp air, from twelve to twenty hours; and in this length of time a duck or heron might fly at least six or seven hundred miles, and would be sure to alight on a pool or rivulet, if blown across sea to an oceanic island or to any other distant point."

The same goes for plants. Any bird that gets its feet muddy is also going to get them covered in seeds:

"I do not believe that botanists are aware how charged the mud of ponds is with seeds: I have tried several little experiments, but will here give only the most striking case: I took in February three table-spoonfuls of mud from three different points, beneath water, on the edge of a little pond; this mud when dry weighed only 6 3/4 ounces; I kept it covered up in my study for six months, pulling up and counting each plant as it grew; the plants were of many kinds, and were altogether 537 in number; and yet the viscid mud was all contained in a breakfast cup!"

Researchers are still doing this sort of experiment. Bland Finlay and Ken Clarke took 25.2 microlitres of sediment from Priest Pot, a pond in Cumbria, England, and found that it contained 32 out of 41 species recorded worldwide of the protozoan genus Paraphysomonas. Finlay, the last time I spoke to him, thought that microbes are so mobile that they can disperse to any place that can support them -- the 'everything is everywhere' school of microbial ecology. Others, using evidence from heterogeneity in microbes' DNA sequences, think differently.


The bulk of this chapter, though, is taken up with explaining why islands contain the species they do. Here, Darwin hits the bullseye time and again:

"The species of all kinds which inhabit oceanic islands are few in number compared with those on equal continental areas"

What kinds of species do you find? The most mobile. He's always looking for an opening to land a jab on the "theory of creation":

"Why, it may be asked, has the supposed creative force produced bats and no other mammals on remote islands? On my view this question can easily be answered; for no terrestrial mammal can be transported across a wide space of sea, but bats can fly across."

And what role does evolution play in creating island nature?

"Although in oceanic islands the number of kinds of inhabitants is scanty, the proportion of endemic species (i.e. those found nowhere else in the world) is often extremely large... This fact might have been expected on my theory for, as already explained, species occasionally arriving after long intervals in a new and isolated district, and having to compete with new associates, will be eminently liable to modification, and will often produce groups of modified descendants."

"Oceanic islands are sometimes deficient in certain classes, and their places are apparently occupied by the other inhabitants; in the Galapagos Islands reptiles, and in New Zealand gigantic wingless birds, take the place of mammals."

"The most striking and important fact for us in regard to the inhabitants of islands, is their affinity to those of the nearest mainland, without being actually the same species. Numerous instances could be given of this fact. I will give only one, that of the Galapagos Archipelago, situated under the equator, between 500 and 600 miles from the shores of South America. Here almost every product of the land and water bears the unmistakeable stamp of the American continent."

(This is, unsurprisingly, by far the most Galapagosy chapter of the book.)

In 1967, Robert MacArthur and E. O. Wilson formalized many of these insights in their classic book The Theory of Island Biogeography, which described how the number of species on an island depends on the balance between the rate of new arrivals and extinctions, which depends on the island's size and its isolation (this has also been useful in other areas, such as understanding and designing nature reserves, which are effectively islands). But it seems to me that Darwin does much of the groundwork in this chapter.


When Darwin's aim wanders, it can make you say 'Oooooh! So close!'

"[T]here is also a relation, to a certain extent independent of distance, between the depth of the sea separating an island from the neighbouring mainland, and the presence in both of the same mammiferous species or of allied species in a more or less modified condition. Mr. Windsor Earl has made some striking observations on this head in regard to the great Malay Archipelago, which is traversed near Celebes [Sulawesi] by a space of deep ocean; and this space separates two widely distinct mammalian faunas."

What I think he's referring to here is what became known as the Wallace line, which runs between the west coast of Sulawesi and Borneo, and reflects (although does not exactly match) the boundaries of the Eurasian and Australian tectonic plates. "[W]e shall soon have much light thrown on the natural history of this archipelago by the admirable zeal and researches of Mr Wallace," Darwin notes. (Wallace's Malay Archipelago was published in 1872.)

Not knowing about plate tectonics, Darwin explains the discontinuity thus:

"As the amount of modification in all cases depends to a certain degree on the lapse of time, and as during changes of level it is obvious that islands separated by shallow channels are more likely to have been continuously united within a recent period to the mainland than islands separated by deeper channels, we can understand the frequent relation between the depth of the sea and the degree of affinity of the mammalian inhabitants of islands with those of a neighbouring continent."


Finally, and this is something that pops up often through the book, I hadn't realized the impact that invasive species were already having even back in Darwin's day.

"In St. Helena there is reason to believe that the naturalised plants and animals have nearly or quite exterminated many native productions."

"[F]rogs have been introduced into Madeira, the Azores, and Mauritius, and have multiplied so as to become a nuisance."

What's Darwin 3,000 AD going to see? The way things are going, rats, goats, zebra mussels, kudzu vine, and a bunch of other generalist, opportunist species, regardless of where on Earth the interstellar cruiser Beagle touches down. Will he or she be as good at Darwin 1.0 at understanding how things got that way, and why?

On Monday, what looks like a right old ragbag: Mutual affinities of organic beings: Morphology: Embryology: Rudimentary Organs.


More like this

In my neighborhood in upstate New York, those zebra mussels have been spreading from lake to lake, presumably hitching rides on pleasure craft. No doubt, the water lines in water-cooled boat motors are modern equivalents of bird gizzards. They have proliferated enormously, becoming a nuisance in many ways. For example, some folks I know have to hire a diver every spring to clean the mussels out of the water intake for their lakeside summer cottage.

One unusual by-product of zebra mussel infestation in many lakes is that, because the mussels are vigorous filter-feeders, the water has become considerably clarified. You can often see the bottom even several meters down in previously murky lakes. Undoubtedly, this change has great implications for the ecology of the lake dwellers, and it is interesting to contemplate how natural selection may respond.

By Richard Blaine (not verified) on 14 Feb 2009 #permalink

It would be interesting to speculate on the impact of zebra mussel as well as other species of plants and animals which use the same tactic to relocate and spread. There are plants which have prickly seeds, i do not recall the name, which are native to Eastern Washington and spread by catching onto hikers' socks. what i speculate on is that, are there or have there been a species of plant which uses this tactic to populate and eventually consume an area which it has been dropped off at.

By Bill Locke (not verified) on 04 Jan 2012 #permalink