Chap. III. 
MUTUAL CHECKS TO INCREASE. 
73 
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 Stafford-^ 
shire, 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 lately tried, I have found that the visits 
of bees are necessary for the fertilisation of some kinds 
of clover; but humble-bees alone visit the red clover 
(Trifolium pratense), as other bees cannot reach the 
nectar. Hence I have very little doubt, that if the 
E 
