grow plentifully in the Craven of Yorkshire down to 600 or 800 feet
above the sea, while in Snowdon it dislikes growing lower than 2000
feet, and is not plentiful even there?--it will reply--Because in
the Craven I can get as much carbonic acid as I want from the
decomposing limestone; while on the Snowdon Silurian I get very
little; and I have to make it up by clinging to the mountain tops,
for the sake of the greater rainfall. But if you ask Polypodium
calcareum--How is it you choose only to grow on limestone, while
Polypodium Dryopteris, of which, I suspect, you are only a variety,
is ready to grow anywhere?--Polypodium calcareum will refuse, as
yet, to answer a word.

Again--I can only give you the merest string of hints--you will find
in your questionings that many plants and animals have no reason at
all to show why they should be in one place and not in another, save
the very sound reason for the latter which was suggested to me once
by a great naturalist. I was asking--Why don't I find such and such
a species in my parish, while it is plentiful a few miles off in
exactly the same soil?--and he answered--For the same reason that
you are not in America. Because you have not got there. Which
answer threw to me a flood of light on this whole science. Things
are often where they are, simply because they happen to have got
there, and not elsewhere. But they must have got there by some
means, and those means I want young naturalists to discover; at
least, to guess at.

A species, for instance--and I suspect it is a common case with
insects--may abound in a single spot, simply because, long years
ago, a single brood of eggs happened to hatch at a time when eggs of
other species, who would have competed against them for food, did