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?