CHAP. III.] STRUGGLE FOR EXISTENCE. 53 



the parasitic insects would probably increase ; and this would 

 lessen the number of the navel-frequenting flies then cattle and 

 horses would become feral, and this would certainly greatly alter 

 (as indeed I have observed in parts of South America) the vegeta- 

 tion : this again would largely affect the insects ; and this, as we 

 have just seen in Staffordshire, the insectivorous birds, and so 

 onwards in ever-increasing 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 a 

 web 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 experiments that humble-bees are almost indis- 

 pensable to the fertilisation 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 pro- 

 duced not one. Again, 100 heads of red clover (T. pratense) pro- 

 duced 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 Col. 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 Col. Newman says, " Near villages and 



