ompeted against them for food, did not hatch;
and they may remain confined to that spot, though there is plenty of good
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 both grow 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 the