pter
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 Ecuador, 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
of that genus tend to vary?

I have inquired of Sclater and he tells me that he has a considerable
distrust of Gould's information on this point, but that he has not
himself studied humming-birds.

In regard to shells, I have always found that dealers have a positive
prejudice against intermediate forms, and one of the most philosophical
of them, now no more, once confessed to me that it was very much against
his trade interest to give an honest opinion that certain varieties were
not real species, or that certain forms, made distinct genera by some
conchologists, ought not so to rank. Nine-tenths of his customers, if
told that it was not a good genus or good species, would say, "Then I
need not buy it." What they wanted was names, not things. Of course
there are genera in which the species are much better defined than in
others, but you would explain this, as Darwin and Hooker do, by the
greater length of time during which they have existed, or the greater
activity of changes, organic and inorganic, which have taken place in