II 
TIIK STRUGGLE FOR EXISTENCE 
19 
was greater than is often seen in passing from one quite 
different soil to another. Besides a great change in the pro¬ 
portional numbers of the native heath-plants, twelve species 
which could 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 Rengger 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 otlu-r 
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 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 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.” 1 
1 The Origin of Species, p. 56. 
