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