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SIR C. LYELL TO A.R. WALLACE


_73 Harley Street. May 2, 1867._

My dear Sir,--I forgot to ask you last night about an ornithological
point which I have been discussing with the Duke of Argyll. In Chapter
V. of his "Reign of Law" (which I should be happy to lend you, if you
have time to look at it immediately) he treats of humming-birds, saying
that Gould has made out about 400 species, every one of them very
distinct from the other, and only one instance, in Ecuadór, of a species
which varies in its tail-feathers in such a way as to make it doubtful
whether it ought to rank as a species, an opinion to which Gould
inclines, or only as a variety or incipient species, as the Duke thinks.
For the Duke is willing to go so far towards the transmutation theory as
to allow that different humming-birds may have had a common ancestral
stock, provided it be admitted that a new and marked variety appears at
once with the full distinctness of sex so remarkable in that genus.

According to his notion, the new male variety and the female must both
appear at once, and this new race or species must be regarded as an
"extraordinary birth." My reason for troubling you is merely to learn,
since you have studied the birds of South America, and I hope collected
some humming-birds, whether Gould is right in saying that there are so
many hundred very distinct species without instances of marked varieties
and transitional forms. If this be the case, would it not present us
with an exception to the rule laid down by Darwin and Hooker that when a
genus is largely represented in a continuous tract of land the species