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 larvæ 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 the ground beyond them already occupied by other plants, who will not tolerate a fresh mouth, having only just enough to feed themselves.  Take the case of Saxifraga hypnoides and S. umbrosa, “London pride.”  They are two especially strong species.  They show that, S. hypnoides especially, by their power of sporting, of diverging into varieties; they show it equally by their power of thriving anywhere, if they can only get there.  They will grow both in my sandy garden, under a rainfall of only 23 inches, more luxuriantly than in their native mountains under a rainfall of 50 or 60 inches.  Then how is it that S. hypnoides cannot get down off the mountains; and that S. umbrosa, though in Kerry it has got off the mountains and down to the sea-level, exterminating, I suspect, many species in its progress, yet cannot get across County Cork?  The only answer is, I believe, that both species are continually trying to go ahead; but that the other plants already in front of them are too strong for them, and massacre their infants as soon as born.

And this brings us to another curious question: the sudden and abundant appearance of plants, like the foxglove and Epilobium angustifolium, in spots where they have never been seen before.  Are there seeds, as some think, dormant in the ground; or are the seeds which have germinated, fresh ones wafted thither by wind or otherwise, and only able to germinate in that one spot because there the soil is clear?  General Monro, now famous for his unequalled memoir on the bamboos, holds to the latter theory.  He pointed out to me that the Epilobium seeds, being feathered could travel with the wind; that the plant always made its appearance first on new banks, landslips, clearings, where it had nothing to compete against; and that the foxglove did the same.  True, and most painfully true, in the case of thistles and groundsels: but foxglove seeds, though minute, would hardly be carried by the wind any more than those of the white clover, which comes up so abundantly in drained fens.  Adhuc sub judice lis est, and I wish some young naturalists would work carefully at the solution; by experiment, which is the most sure way to find out anything.

But in researches in this direction they will find puzzles enough.  I will give them one which I shall be most thankful to hear they have solved within the next seven years—How is it that we find certain plants, namely, the thrift and the scurvy grass, abundant on the sea-shore and common on certain mountain-tops, but nowhere between the two?  Answer me that.  For I have looked at the fact for years—before, behind, sideways, upside down, and inside out—and I cannot understand it.

But all these questions, and especially, I suspect, that last one, ought to lead the young student up to the great and complex question—How were these islands re-peopled with plants and animals, after the long and wholesale catastrophe of the glacial epoch?

I presume you all know, and will agree, that the whole of these islands, north of the Thames, save certain ice-clad mountain-tops, were buried for long ages under an icy sea.  From whence did vegetable and animal life crawl back to the land, as it rose again; and cover its mantle of glacial drift with fresh life and verdure?

Now let me give you a few prolegomena on this matter.  You must study the plants of course, species by species.  Take Watson’s “Cybele Britannica” and Moore’s “Cybele Hibernica;” and let—as Mr. Matthew Arnold would say—“your thought play freely about them.”  Look carefully, too, in the case of each species, at the note on its distribution, which you will find appended in Bentham’s “Handbook,” and in Hooker’s “Student’s Flora.”  Get all the help you can, if you wish to work the subject out, from foreign botanists, both European and American; and I think that, on the whole, you will come to some such theory as this for a general starling platform.  We do not owe our flora—I must keep to the flora just now—to so many different regions, or types, as Mr. Watson conceives, but to three, namely, an European or Germanic flora, from the south-east; an Atlantic flora, from the south-east; a Northern flora, from the north.  These three invaded us after the glacial epoch; and our general flora is their result.

But this will cause you much trouble.  Before you go a step farther you will have to eliminate from all your calculations most of the plants which Watson calls glareal, i.e. found in cultivated ground about habitations.  And what their limit may be I think we never shall know.  But of this we may be sure; that just as invading armies always bring with them, in forage or otherwise, some plants from their own country—just as the Cossacks, in 1815, brought more than one Russian plant through Germany into France—just as you have already a crop of North German plants upon the battle-fields of France—thus do conquering races bring new plants.  The Romans, during their 300 or 400 years of occupation and civilisation, must have brought more species, I believe, than I dare mention.  I suspect them of having brought, not merely the common hedge elm of the south, not merely the three species of nettle, but all our red poppies, and a great number of the weeds which are common in our cornfields; and when we add to them the plants which may have been brought by returning crusaders and pilgrims; by monks from every part of Europe, by Flemings or other dealers in foreign wool—we have to cut a huge cantle out of our indigenous flora: only, having no records, we hardly know where and what to cut out; and can only, we elder ones, recommend the subject to the notice of the younger botanists, that they may work it out after our work is done.

Of course these plants introduced by man, if they are cut out, must be cut out of only one of the floras, namely, the European; for