88 DARWINISM STATED BY DARWIN HIMSELF. 



INFLUENCE OF INSECTS IN THE STRUGGLE FOE EXIST- 

 ENCE. 



In several parts of the world insects deter- 

 mine 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 

 Azaraand 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 ha- 

 bitually checked by some means, probably by other para- 

 sitic 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 vegetation : 

 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 recur- 

 ring with varying success ; and yet in the long run the 

 forces are so nicely balanced that the face of Nature re- 

 mains 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 deso- 

 late the world, or invent laws on the duration of the 

 forms of life ! 





