The promised more on Janet Browne’s Darwin biography. A couple of sentences down on the same page (page 141, to be exact):

And when Sedgwick arrived he tried to entertain him in an appropriately geological fashion by telling him of the gravel pits near Shrewsbury. But Darwin’s story of the labourer who found a tropical shell in the gravel brought only a peal of laughter and the remark that this could not be true. If the shell were genuinely embedded there, said Sedgwick, it would overthrow everything that was known about the superficial deposits of the Midland counties…Recounting the story later, Darwin remembered being astonished that Sedgwick was not more delighted by his strange fact. ‘Nothing before had ever made me thoroughly realise, though I had read various scientific books, that science consists in grouping facts so that general laws or conclusions may be drawn from them.’ What Sedgwick went on to explain to him was that there must be a great deal of mutually supportive material for scientific theories of all denominations. Once such theories were established, it took more than an isolated shell to change them.

A simple point, but interesting, I think. Interesting partly that he hadn’t realized it before, and that one incident made it so clear to him. ‘And from the way Darwin continued to hold this salutory episode in mind,’ Browne goes on to say, ‘it evidently had a marked effect on his scientific practice.’ One shell, one story, one peal of laughter. So learning takes place.

And 142-143. They are on the field trip. According to Greenough’s map, there should be Old Red Sandstone underlying an escarpment where Sedgwick saw no sign of it. He sent Darwin to search for signs on one side while he searched on the other.

On his own for the first time since leaving Shrewsbury, Darwin could not find any trace of the desired rock. He was more than a little anxious by the time he returned to Sedgwick, because it was easy to miss details in the field and hard to contradict an acknowledged authority like Greenough. He had scoured the countryside for elusive corroborative signs. Yet Sedgwick was very pleased with him…explaining how his researches would require the revision of a major portion of the national map. Sedgwick too had not seen ‘a particle’ of Old Red…[F]ew professors would have accepted a major negative claim like Darwin’s without backtracking to check on the data.

I like that because it’s the black swan thing, and because it made Darwin anxious. The black swan makes for instance UN weapons inspectors anxious, too, because however hard they look they can’t know, and they know they can’t know, that they have searched everywhere. In fact in that case they know perfectly well they haven’t.

His chagrin at Sedgwick’s brusque response to the tropical shell in the gravel pit was transformed into a fleeting but thoroughly practical awareness of the philosophical structure of science. He went on his way to Barmouth with his wits sharpened and with a good deal more intellectual purpose…

Interesting, don’t you think?