A recent New York Times article illustrates how chance acts as a imitation or placeholder cause in scientific explanation. The article recounts a study that compares the survival of the dinosaurs to that of another reptile group:
But then at the end of the Triassic, for some unknown reason the dinosaurs survived while almost all the crurotarsans did not. “There was a certain amount of luck involved,” Dr. Benton said. “One group got pretty much wiped out and another group soldiered on and took off. The dinosaurs finally got their chance.”
Notice the phrase "for some unknown reason." In other words, "luck" takes the place of an actual explanation for the extinction of one species and the survival of another.
I don't have access to the actual paper on which the Times article is based, but its abstract puts it in scientifically more mellifluous terms:
The results strongly suggest that historical contingency, rather than prolonged competition or general "superiority," was the primary factor in the rise of dinosaurs.
"Historical contingency." It's translation as "luck" is faithful to the thoughts of the researchers. Steve Brusatte explains, "Why did crurotarsans go extinct and not dinosaurs? We don't know the answer to that, but we suspect that it was nothing more than luck, plain and simple." (Bristol University press release)
"Nothing more than luck".
How did they come to this conclusion? The Times writes:
“The assumption is that the diversity or range of body forms is more or less proportional to the number of modes of life that they’d occupy,” Dr. Benton said. So the finding shows that the crurotarsans were more diverse in terms of their lifestyle, diet and habitat — they filled more ecological niches and were, if anything, the more successful of the two groups in the late Triassic. “The dinosaurs didn’t find a way to squeeze into the crurotarsans’ role,” he said.
The press release put it thusly:
[C]rurotarsans were more abundant (more individuals, more fossils, more species) than dinosaurs in many Triassic ecosystems, and crurotarsans were in some cases more diverse (greater number of species). Putting all this together, it is very difficult to argue that dinosaurs were ‘superior’ to crurotarsans, or that they were out-competing crurotarsans.
So, in other words, 'We can't explain it with our brute quantitative measure of "superiority," so the only explanation can be "chance".' True enough.
But having to resort to chance only shows the coarseness of scientific measures, and not the actual reality of the situation. It's kind of like failing to catch any fish in a lake with a net with three-inch-wide mesh and then proclaiming the absence of minnows. Chance cannot be a final explanation.
Henry Fountain, "Dinosaurs Got by With a Little Bit of Luck," New York Times (September 12, 2008).
"Dinosaurs' 'superiority' challenged by their crocodile cousins," Bristol University Press release (11 September 2008).
Stephen L. Brusatte, Michael J. Benton, Marcello Ruta, Graeme T. Lloyd Science, "Superiority, Competition, and Opportunism in the Evolutionary Radiation of Dinosaurs," "" Science 321:5895 (12 September 2008) pp. 1485-1488.
Maybe I'm running out of steam, or maybe I just had too many distractions this summer.