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“The shape of the fragments of stone at the base of our precipice may be called accidental, but this is not strictly correct; for the shape of each depends on a long sequence of events, all obeying natural laws.... But in regard to the use to which the fragments may be put, their shape may be strictly said to be accidental....”
In his article in the Nation (March 19th, 1868), Asa Gray criticised the metaphor as follows:—
“But in Mr. Darwin’s parallel, to meet the case of nature according to his own view of it, not only the fragments of rock (answering to variation) should fall, but the edifice (answering to natural selection) should rise, irrespective of will or choice!”
This passage is quoted in the “Life and Letters” (Vol. III., p. 84), and Francis Darwin makes the convincing reply:—
“But my father’s parallel demands that natural selection shall be the architect, not the edifice—the question of design only comes in with regard to the form of the building materials.”
Darwin’s reply was contained in his letter to Asa Gray dated May 8th, 1868:—
“You give a good slap at my concluding metaphor: undoubtedly I ought to have brought in and contrasted natural and artificial selection; but it seemed so obvious to me that natural selection depended on contingencies even more complex than those which must have determined the shape of each fragment at the base of my precipice. What I wanted to show was that, in reference to pre-ordainment, whatever holds good in the formation of an English pouter-pigeon holds good in the formation of a natural species of pigeon. I cannot see that this is false. If the right variations occurred, and no others, natural selection would be superfluous.”
To this, Asa Gray replied in his letter of May 25th:—
“As to close of my article, to match close of your book,—you see plainly I was put on the defence by your reference to an old hazardous remark of mine. I found your stone-house argument unanswerable in substance (for the notion of design must after all rest mostly on faith, and on accumulation of adaptations, &c.); so all I could do was to find a vulnerable spot in the shaping of it, fire my little shot, and run away in the smoke.