Not that
under nature the relations will ever be as simple as this. Battle within
battle must be continually recurring with varying success; and yet in the
long-run the forces are so nicely balanced that the face of nature remains
for long periods of time uniform, though assuredly the merest trifle would
give the victory to one organic being over another. Nevertheless, so
profound is our ignorance, and so high our presumption, that we marvel when
we hear of the extinction of an organic being; and as we do not see the
cause, we invoke cataclysms to desolate the world, or invent laws on the
duration of the forms of life!
I am tempted to give one more instance showing how plants and animals,
remote in the scale of nature, are bound together by a web of complex
relations. I shall hereafter have occasion to show that the exotic Lobelia
fulgens is never visited in my garden by insects, and consequently, from
its peculiar structure, never sets a seed. Nearly all our orchidaceous
plants absolutely require the visits of insects to remove their
pollen-masses and thus to fertilise them. I find from experiments that
humble-bees are almost indispensable to the fertilisation of the heartsease
(Viola tricolor), for other bees do not visit this flower. I have also
found that the visits of bees are necessary for the fertilisation of some
kinds of clover; for instance twenty heads of Dutch clover (Trifolium
repens) yielded 2,290 seeds, but twenty other heads, protected from bees,
produced not one.
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