Chap. III. | STRUGGLE FOR EXISTENCE. 57 



insectivorous birds were to decrease in Paraguay, 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 obser- 

 ved 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-increasing 

 circles of complexity. Not that under nature the relations will 

 ever be as simple as this. Battle within battle must be con- 

 tinually 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. Never- 

 theless, 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 thai 

 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 indispensable 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 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 on the 

 number of field-mice, which destroy their combs and nests ; and 

 CoL Newman, who has long attended to the habits of humble-bees, 



