Seite - 87 - in The Origin of Species

STRUGGLE FOR EXISTENCE 87 then cattle and horses would become feral, and this would certainly greatly alter (as indeed I have observeu in parts of South America) the vegetation: this again would largely affect the insects; and this, as we have just seen in Stafford- shire, the insectivorous birds, and so onwards in ever-in- creasing circles of complexity. Not that under nature the relations will ever be as simple as this. Battle within battle must be continually recurring with varying success; and yet in the long-run the forces are so nicely balanced, that the face of nature remains for long periods of time uniform, though assuredly the merest trifle would 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,remote in the scale of nature,are bound together by aweb 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 experi- ments that humble-bees are almost indispensable to the fer- tilisation 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, 20 heads of Dutch clover (Trifolium repens) yielded 2,290 seeds, but 20 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 suflkient to depress the wing petals. Hence we may infer as highly probable that, if the whole genus of humble-bees