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 larvae 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.