STRUGGLE FOR EXISTENCE 57 



the existence of cattle. Perhaps Paraguay offers the most curious 

 instance of this; for here neither cattle nor horses nor dogs have 

 ever run wild, though they swarm southward and northward in a 

 feral state; and Azara and Rengger have shown that this is 

 caused by the greater number in Paraguay of a certain fly, which 

 lays its eggs in the navels of these animals when first born. The 

 increase of these flies, numerous as they are, must be habitually 

 checked by some means, probably by other parasitic insects. 

 Hence, if 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 certainly greatly alter 

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

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

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

 onward 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 as- 

 suredly 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 al- 

 most indispensable 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 (Tri- 

 folium repens) yielded 2,290 seeds, but twenty other heads, pro- 

 tected 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. 



