Africa,
where large herbivorous mammals abound. Now, if the long-continued
presence of these mammals is a factor in the production of spines by
Natural Selection, they should be wholly or comparatively absent in
regions equally arid where there are no mammals. The Galapagos seem to
be such a case--also perhaps some of the Sandwich Islands, and generally
the extra-tropical volcanic islands. Also Australia comparatively, and
the highlands of Madagascar.

Of course, the endemic species must be chiefly considered, as they have
had time to be modified by the conditions. If you can give me the facts,
or your general impression from your study of these floras, I shall be
much obliged. I see, of course, many other objections to Geddes's
theory, but this seems to offer a crucial test.--Believe me yours very
truly,

ALFRED R. WALLACE.

* * * * *

TO DR. W.B. HEMSLEY

_Frith Hill, Godalming. September 13, 1888._

Dear Mr. Hemsley,--Many thanks for your interesting letter. The facts
you state seem quite to support the usual view, that thorns and spines
have been developed as a protection against other animals. The few spiny
plants in New Zealand may be for protection against land molluscs, of
which there are several species as large as any in the tropics. Of
course in Australia we should expect only a comparative scarcity of
spines, as there are many herbivorous marsupials in the
country.--Believe me yours very faithfully,

ALFRED R. WALLACE.

* * * * *

The next and several of the succeeding letters refer to the translations
of Weismann's "Essays upon Heredity and Kindred Biological Problems"
(Oxford, 1889), and to "Darwinism" (London, 1889).

TO PROF. POULTON

_Frith Hill, Godalming. November 4, 1888._

My dear Mr. Poulton,--I returned you the two first of Weismann's essays,
with a few notes and corrections in pencil on that on "Duration of
Life." Looking over some old papers, I have just come across a short
sketch on two pages, on "The Action of Natural Selection in producing
Old Age, Decay and Death," written over twenty years ago.[16] I had the
same general idea as Weismann, but not that beautiful suggestion of the
duration of life, in each case, being the _minimum_ necessary for the
preservation of the species. _That_ I think masterly. The paper on
"Heredity" is intensely interesting, and I am waiting anxiously for the
concluding part. I wil