11 THE STRUGGLE FOR EXISTENCE 19 



was greater than is often seen in passing from one quite 

 difierent soil to another. Besides a great change in the jDro- 

 portional numbers of the native heath-plants, twelve species 

 which coidd not be found on the heath flourished in the 

 plantations. The effect on the insect life must have been still 

 greater, for six insectivorous birds which were very common 

 in the plantations were not to be seen on the heath, which 

 was, however, frequented by two or three different species of 

 insectivorous birds. It would have required continued study 

 for several years to determine all the differences in the 

 organic life of the two areas, but the facts stated by Mr. 

 Darwin are sufficient to show how great a change may be 

 effected by the introduction of a single kind of tree and the 

 keeping out of cattle. 



The next case I will give in Mr. Darwin's own words : 

 " In several parts of the world 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 numbers, 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 Avould probably 

 increase ; and this would lessen the number of the navel- 

 frequenting flies — then cattle and horses would become feral, 

 and this would greatly alter (as indeed I have observed in 

 parts of South America) the vegetation : this again would 

 largely affect the insects, and this, as Ave have just seen in 

 Staffordshire, the insectivorous l)irds, 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 a long time uniform, though assuredly 

 the merest trifle would give the victory to one organic being 

 over another."^ 



' The Origin of Specirs, \\. fiG. 



