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 starting 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-west; 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 further 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 they, probably, came from the south-east, by whatever means they came.