THE ORIGIN OF SPECIES. 41 



southward or in descending a mountain. "When we reach the 

 arctic regions, or snow-capped summits, or absolute deserts, the 

 struggle for life is almost exclusively with the elements. 



" That climate acts in main part indirectly by favoring other 

 species, we may clearly see in the prodigious number of plants 

 in our gardens which can perfectly well endure our climate, but 

 which never become naturalized, for they cannot compete with 

 our native plants, nor resist destruction by our native animals." 

 (pp. 68, 69.) 



After an instructive instance in which " cattle ab- 

 solutely determine the existence of the Scotch fir," 

 we are referred to cases in which insects determine 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 Eengger 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 in- 

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

 checked by some means, probably by birds. Hence, if certain 

 insectivorous birds (whose numbers are probably regulated by 

 hawks or beasts of prey) were to increase in Paraguay, the flies 

 would decrease 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 haye just seen in 

 Staffordshire, the insectivorous birds, and so onward in ever- 

 increasing circles of complexity. "We began tliis series by in- 

 sectivorous birds, and we had 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 as- 

 suredly the merest trifle would often give the victory to one 

 organic being over another. Nevertheless, so profound is our 



