That European flora invaded us, I presume, immediately after the glacial epoch, at a time when France and England were united, and the German Ocean a mere network of rivers, which emptied into the deep sea between Scotland and Scandinavia.  And here I must add, that endless questions of interest will arise to those who will study, not merely the invasion of that truly European flora, but the invasion of reptiles, insects, and birds, especially birds of passage, which must have followed it as soon as the land was sufficiently covered with vegetation to support life.  Whole volumes remain to be written on this subject.  I trust that some of your younger members may live to write one of them.  The way to begin will be; to compare the flora and fauna of this part of England very carefully with that of the southern and eastern counties; and then to compare them again with the fauna and flora of France, Belgium, and Holland.

As for the Atlantic flora, you will have to decide for yourselves whether you accept or not the theory of a sunken Atlantic continent.  I confess that all objections to that theory, however astounding it may seem, are outweighed in my mind by a host of facts which I can explain by no other theory.  But you must judge for yourselves; and to do so you must study carefully the distribution of heaths both in Europe and at the Cape, and their non-appearance beyond the Ural Mountains, and in America, save in Labrador, where the common ling, an older and less specialised form, exists.  You must consider, too, the plants common to the Azores, Portugal, the West of England, Ireland, and the Western Hebrides.  In so doing young naturalists will at least find proofs of a change in the distribution of land and water, which will utterly astound them when they face it for the first time.

As for the Northern flora, the question whence it came is puzzling enough.  It seems difficult to conceive how any plants could have survived when Scotland was an archipelago in the same ice-covered condition as Greenland is now; and we have no proof that there existed after the glacial epoch any northern continent from which the plants and animals could have come back to us.  The species of plants and animals common to Britain, Scandinavia, and North America, must have spread in pre-glacial times when a continent joining them did exist.

But some light has been thrown on this question by an article, as charming as it is able, on “The Physics of the Arctic Ice,” by Dr. Brown of Campster.  You will find it in the “Quarterly Journal of the Geological Society” for February, 1870.  He shows there that even in Greenland peaks and crags are left free enough from ice to support a vegetation of between three hundred or four hundred species of flowering plants; and, therefore, he well says, we must be careful to avoid concluding that the plant and animal life on the dreary shores or mountain-tops of the old glacial Scotland was poor.