Arctic regions. Such
plants seem exposed to such much greater difficulties in diffusion. Very
many thanks for all your kindness and answers to my questions.

P.S.--If anything should occur to you on variability of naturalised or
agrarian plants, I hope that you will be so kind as to let me hear, as
it is a point which interests me greatly.

LETTER 330. ASA GRAY TO CHARLES DARWIN. Cambridge, Mass., September
23rd, 1856.

Dr. Engelmann, of St. Louis, Missouri, who knew European botany well
before he came here, and has been an acute observer generally for twenty
years or more in this country, in reply to your question I put to him,
promptly said introduced plants are not particularly variable--are not
so variable as the indigenous plants generally, perhaps.

The difficulty of answering your questions, as to whether there are any
plants social here which are not so in the Old World, is that I know so
little about European plants in nature. The following is all I have
to contribute. Lately, I took Engelmann and Agassiz on a botanical
excursion over half a dozen miles of one of our seaboard counties; when
they both remarked that they never saw in Europe altogether half so much
barberry as in that trip. Through all this district B. vulgaris may be
said to have become a truly social plant in neglected fields and copses,
and even penetrating into rather close old woods. I always supposed that
birds diffused the seeds. But I am not clear that many of them touch the
berries. At least, these hang on the bushes over winter in the greatest
abundance. Perhaps the barberry belongs to a warmer country than north
of Europe, and finds itself more at home in our sunny summers. Yet out
of New England it seems not to spread at all.

Maruta Cotula, fide Engelmann, is a scattered and rather scarce plant in
Germany. Here, from Boston to St. Louis, it covers the roadsides, and is
one of our most social plants. But this plant is doubtless a native of a
hotter country than North Germany.

St. John's-wort (Hypericum perforatum) is an intrusive weed in all hilly
pastures, etc., and may fairly be called a social plant. In Germany it
is not so found, fide Engelmann.

Verbascum Thapsus is diffused over all the country, is vastly more
common here than in Germany, fide Engelmann.

I suppose Erodium cicutarium was brought to America with cattle from
Spain: it seems to be widely spread over South America out of the
Tropics. In Atlantic U.S. it