that if two or three species...were found to range from America to
India, they would be found to extend through an unusual thickness
of strata--say from the Upper Cretaceous to its lowest bed, or the
Neocomian. Or you may reverse it and say those species which range
throughout the whole Cretaceous, will have wide ranges: viz., from
America through Europe to India (this is one actual case with shells in
the Cretaceous period).

LETTER 16. TO J.D. HOOKER. Down [1845].

I ought to have written sooner to say that I am very willing to
subscribe 1 pound 1 shilling to the African man (though it be murder on
a small scale), and will send you a Post-office-order payable to Kew, if
you will be so good as to take charge of it. Thanks for your information
about the Antarctic Zoology; I got my numbers when in Town on Thursday:
would it be asking your publisher to take too much trouble to send your
Botany ["Flora Antarctica," by J.D. Hooker, 1844] to the Athenaeum Club?
he might send two or three numbers together. I am really ashamed to
think of your having given me such a valuable work; all I can say is
that I appreciate your present in two ways--as your gift, and for its
great use to my species-work. I am very glad to hear that you mean to
attack this subject some day. I wonder whether we shall ever be public
combatants; anyhow, I congratulate myself in a most unfair advantage of
you, viz., in having extracted more facts and views from you than from
any one other person. I daresay your explanation of polymorphism on
volcanic islands may be the right one; the reason I am curious about it
is, the fact of the birds on the Galapagos being in several instances
very fine-run species--that is, in comparing them, not so much one with
another, as with their analogues from the continent. I have somehow
felt, like you, that an alpine form of a plant is not a true variety;
and yet I cannot admit that the simple fact of the cause being
assignable ought to prevent its being called a variety; every variation
must have some cause, so that the difference would rest on our knowledge
in being able or not to assign the cause. Do you consider that a true
variety should be produced by causes acting through the parent? But even
taking this definition, are you sure that alpine forms are not inherited
from one, two, or three generations? Now, would not this be a curious
and valuable experiment (16/1. For an account of work of this character,
see papers