EFFECT OF SETTLEMENT UPON INDIGENOUS VEGETATION. 219 
hundred acres of exactly the same nature had been enclosed twenty- 
five years previously and planted with Scotch fir. The change in 
the native vegetation of the planted part of the heath was most 
remarkable, more than is generally seen in passing from one quite 
different soil to another ; not only the proportional number of the 
heath plants were wholly changed, but twelve species of plants 
(not counting grasses and carices) flourished in the plantations, 
which could not be found on the heath. The effect on the insects. 
must have been still greater, for six insectivorous birds were very 
common in the plantations which were not to be seen on the heath; 
and the heath was frequented by two or three distinct insectivor- 
ous birds. Here we see how potent has been the effect of the 
introduction of a single tree, nothing whatever else having been 
done, with the exception of the land having been enclosed so that. 
cattle could not enter,”’* And again: “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 south- 
ward 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 on 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 navel-frequenting flies—then 
cattle and horses would become feral, and this would certainly 
alter (as indeed I have observed in 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 on in 
ever-increasing circles of complexity.” + 
Another instance of insects determining the existence of mam- 
mals occurs in the Tse-tse districts of Africa, where horses and 
* Origin of Species, 6th Edit., p. 55 et seq. 
+ Ib. p. 56, et seq. 
