y strata? The usual
answer would be, I presume--if we could work it out by twenty years'
experiment, such as Mr. Lawes, of Rothampsted, has been making on the
growth of grasses and leguminous plants in different soils and under
different manures--the usual answer, I say, would be--Because we plants
want such and such mineral constituents in our woody fibre; again,
because we want a certain amount of moisture at a certain period of the
year: or, perhaps, simply because the mechanical arrangement of the
particles of a certain soil happens to suit the shape of our roots and of
their stomata. Sometimes you will get an answer quickly enough;
sometimes not. If you ask, for instance, _Asplenium viride_ how it
contrives to 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 _Polopodium 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 c