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!

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 will concur 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 when
an American forest is cut down, a very different vegetation springs
up; but it has been observed that ancient Indian ruins in the Southern
United States, which must formerly have been cleared of trees, now
display the same beautiful d