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 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 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.