So far, I repeat from him. What are we to conclude? That we
have not materials enough yet, to form any conclusion. The fact stated
by Sauvagiere is not against any law of nature, and is therefore
possible; but it is so little analogous to her habitual processes,
that, if true, it would be extraordinary: that to command our belief,
therefore, there should be such a suite of observations, as that their
untruth would be more extraordinary than the existence of the fact they
affirm. The bark of trees, the skin of fruits and animals, the
feathers of birds, receive their growth and nutriment from the internal
circulation of a juice through the vessels of the individual they cover.
We conclude from analogy, then, that the shells of the testaceous tribe
receive also their growth from a like internal circulation. If it be
urged, that this does not exclude the possibility of a like shell being
produced by the passage of a fluid through the pores of the circumjacent
body, whether of earth, stone, or water; I answer, that it is not within
the usual economy of nature, to use two processes for one species of
production.
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