CflAr. III. STRUGGLE FOR EXISTENCE. 79 



by insectivorous birds, and -we have ended with tlieni. Not 

 tliat in nature the rehitions can ever be as simple as this. Bat- 

 tle within battle must ever be recurring' with varying' success ; 

 and yet in the long-run the forces are so nicely balanced, that 

 the lace of Nature remains uniform for long periods of time, 

 tliough 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 here- 

 after have occasion to show that the exotic Lobelia fulgens, in 

 this part of England, is never visited by insects, and conse- 

 fiucntly, from its peculiar structure, never sets a seed. Near- 

 ly all our orchidaceous plants absolutely require the visits of 

 insects to remove their pollen-masses and thus to fertilize them. 

 I find from experiments that humble-bees are almost indispen- 

 sable to the fertilization of the heart's-ease (Viola tricolor), for 

 other bees do not visit this flower. I have also found that the 

 visits of bees are necessary for the fertilization 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 pro- 

 tected 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 fertilize 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 humljle-bces became extinct or very rare in 

 England, ihe heart's-ease and red clover would become very 

 rare, or wholly disappear. The number of humble-bees in any 

 district depends in a great degree on the number of field-mice, 

 which destroy their combs and nests ; and Colonel Newman, 

 who has long attended lo 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 hirgely depend- 

 ent, as every one knows, on tlic niunber of cats : and Colonel 

 Newman says, " Near villages and small towns I have found 



