um--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
not hatch; and they may remain confined to that spot, though there
is plenty of food for them outside it, simply because they do not
increase fast enough to require to spread out in search of more
food. Thus I should explain a case which I heard of lately of
Anthocera trifolii, abundant for years in one corner of a certain
field, and only there; while there was just as much trefoil all
round for its larvae as there was in the selected spot. I can, I
say, only give hints: but they will suffice, I hope, to show the
path of thought into which I want young naturalists to turn their
minds.

Or, again, you will have to inquire whether the species has not been
prevented from spreading by some natural barrier. Mr. Wallace, whom
you all of course know, has shown in his "Malay Archipelago" that a
strait of deep sea can act as such a barrier between species.
Moritz Wagner has shown that, in the case of insects, a moderately-
broad river may divide two closely-allied species of beetles, or a
very narrow snow-range, two closely-allied species of moths.

Again, another cause, and a most common one, is: that the plants
cannot spread because they find