Charles Darwin › A game
The Loft
Darwin’s whole theory, in two steps. First, breed pigeons the way the London fanciers did. Then take yourself out, and see what is left.
In the 1850s Darwin joined two London pigeon clubs and sat with the fanciers in a gin palace in the Borough. Their fantails carried thirty or forty tail feathers where a pigeon has twelve or fourteen. Their pouters inflated their crops. Their carriers had long beaks, their short-faced tumblers almost none. And every one of them, every fancier knew, was the same species: the rock pigeon of the sea-cliffs.
Your loft holds eight rock pigeons. Pick two to breed. Their chicks will be a little like both parents, and a little different. Pick again. You cannot make anything; you can only choose among what the birds give you.
Try for a fantail (30 tail feathers or more), a pouter (a crop of 1.15 or more), a carrier (beak 1.85 or more) or a short-faced tumbler (beak 0.5 or less). Don’t chase all of them at once. A fancier in Darwin’s library warned that beginners who go for every property at once have their reward by getting nothing.
No man would ever try to make a fantail, till he saw a pigeon with a tail developed in some slight degree in an unusual manner.
— Darwin, On the Origin of Species
The fancier is the only part of the process with intentions, and he is the only part Darwin proposed to remove. Put in his place the ordinary fact that more birds are born than can eat, and that the ones which die are not a random sample.
Here is one flock of rock pigeons, split between two islands. Nobody chooses anything. Each generation three times as many chicks hatch as the island can feed, and only a third survive. On the first island the food is easiest to reach with a longer beak; on the second, with a shorter one. That is all.
Island one
Food deep in husks: a longer beak gets more.
Island two
Small seeds on bare ground: a shorter beak gets more.
Each dot is a bird; the picture is the average bird; the line underneath is the average beak over time.
One flock, two kinds of bird
Nobody chose. The birds were not trying. All that happened was that more were born than could eat, and the ones that died were not a random sample. Tick the box and run it again: if who dies is pure chance, the beaks wander a little and go nowhere.
Darwin’s problem for years was why the descendants of one form should spread apart into many, rather than simply change along one line. His answer came to him in his carriage: the modified offspring of a successful form tend to fill many different places in the economy of nature. So there is a standing advantage in being unlike your relatives. So life comes in branches.
Now run it not for the lifetime of a fancy, but for the whole length of the geological record.
The Galapagos could show him the result. Only the pigeon clubs could show him the process, because the fanciers were doing it on purpose, in public, in a shed, and you could watch.
— from the book
The birds, the numbers and the islands here are an illustration, not a model of any real flock. The fanciers, the breeds and the argument are Darwin’s, from the book.
More about Darwin
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