64 STRUGGLE FOR EXISTENCE. 



certain 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 cer- 

 tainly greatly alter (as indeed I have observed 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 onward in ever-increas- 

 ing 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 folgens 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 fertilize them. I 

 find from experiments that humble-bees are almost indispens- 

 able 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, twenty heads of Dutch 

 clover (Trifolium repens) yielded 2,290 seeds, but twenty 

 other heads, protected from bees, produced not one. Again, 

 one hundred 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 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 

 inamble-bees became extinct or very rare in England, the 



