CHAPTER III. STRUGGLE FOR EXISTENCE.
5. COMPLEX RELATIONS OF ALL ANIMALS AND PLANTS TO EACH OTHER IN THE STRUGGLE FOR EXISTENCE. (continued)
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. Again, 100 heads of red clover (T. pratense) produced
2,700 seeds, but the same number of protected heads produced not a single
seed. Humble bees alone visit red clover, as other bees cannot reach the
nectar. It has been suggested that moths may fertilise the clovers; but I
doubt whether they could do so in the case of the red clover, from their
weight not being sufficient to depress the wing petals. Hence we may infer
as highly probable that, if the whole genus of humble-bees became extinct
or very rare in England, the heartsease and red clover would become very
rare, or wholly disappear. The number of humble-bees in any district
depends in a great measure upon the number of field-mice, which destroy
their combs and nests; and Colonel Newman, who has long attended to the
habits of humble-bees, believes that "more than two-thirds of them are thus
destroyed all over England." Now the number of mice is largely dependent,
as every one knows, on the number of cats; and Colonel Newman says, "Near
villages and small towns I have found the nests of humble-bees more
numerous than elsewhere, which I attribute to the number of cats that
destroy the mice." Hence it is quite credible that the presence of a
feline animal in large numbers in a district might determine, through the
intervention first of mice and then of bees, the frequency of certain
flowers in that district!
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