invent laws on the duration of the forms of life!
I am tempted to give one more instance showing how plants and animals,
most 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, in this part of England, is never visited by
insects, and consequently, from its peculiar structure, never can set a
seed. Many of our orchidaceous plants absolutely require the visits of
moths to remove their pollen-masses and thus to fertilise them. I have,
also, reason to believe that humble-bees are indispensable to the
fertilisation of the heartsease (Viola tricolor), for other bees do not
visit this flower. From experiments which I have tried, I have found
that the visits of bees, if not indispensable, are at least highly
beneficial to the fertilisation of our clovers; but humble-bees alone
visit the common red clover (Trifolium pratense), as other bees cannot
reach the nectar. Hence I have very little doubt, 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 degree on the number of field-mice, which
destroy their combs and nests; and Mr. H. 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 Mr.
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!
In the case of every species, many different checks, acting at
different periods of life, and during different seasons or years,
probably come into play; some one check or some few being generally the
most potent, but all concurring in determining the average number or
even the existence of the species. In some cases it can be shown that
widely-different checks act on the same species in different districts.
When we look at the plants and bushes clothing an entangled bank, we
are tempted to attribute their proportional numbers and kinds to what
we call chance. But how false a view is this! Every one has heard that
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