by some means, probably by birds. Hence, if
certain insectivorous birds (whose numbers are probably regulated by
hawks or beasts of prey) were to increase in Paraguay, the flies would
decrease--then cattle and horses would become feral, and this would
certainly greatly alter (as indeed I have observed in parts of South
America) the vegetation: this again would largely affect the insects;
and this, as we just have seen in Staffordshire, the insectivorous
birds, and so onwards in ever-increasing circles of complexity. We began
this series by insectivorous birds, and we have ended with them. Not
that in nature the relations can ever be as simple as this. Battle
within battle must ever be recurring with varying success; and yet in
the long-run the forces are so nicely balanced, that the face of nature
remains uniform for long periods of time, though assuredly the merest
trifle would often 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,
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; an