Chap. III. MUTUAL CHECKS TO INCREASE. 73 



indeed I have observed in parts of South America) the 

 vegetation : tins 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 insecti- 

 vorous 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 (Yiola 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 



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