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.
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