44 
DISTRIBUTION OF ANIMALS. 
[PART I. 
different vegetation and a considerably modified fauna to occupy 
the country. 
Organic Changes as affecting Distribution . — We have now briefly 
touched on some of the direct effects of changes in physical 
geography, climate, and vegetation, on the distribution of ani- 
mals ; but the indirect effects of such changes are probably of 
quite equal, if not of greater importance. Every change 
becomes the centre of an ever-widening circle of effects. The 
different members of the organic world are so bound together by 
complex relations, that any one change generally involves 
numerous other changes, often of the most unexpected kind. 
We know comparatively little of the way in which one animal 
or plant is bound up with others, but we know enough to assure 
us that groups the most apparently disconnected are often 
dependent on each other. We know, for example, that the 
introduction of goats into St. Helena utterly destroyed a whole 
flora of forest trees ; and with them all the insects, mollusca, and 
perhaps birds directly or indirectly dependent on them. Swine, 
which ran wild in Mauritius, exterminated the Dodo. The same 
animals are known to be the greatest enemies of venomous 
serpents. Cattle will, in many districts, wholly prevent the 
growth of trees ; and with the trees the numerous insects depen- 
dent on those trees, and the birds which fed upon the insects, 
must disappear, as well as the small mammalia which feed on 
the fruits, seeds, leaves, or roots. Insects again have the most 
wonderful influence on the range of mammalia. In Paraguay a 
certain species of fly abounds which destroys new-born cattle 
and horses ; and thus neither of these animals have run wild in 
that country, although they abound both north and south of it. 
This inevitably leads to a great difference in the vegetation of 
Paraguay, and through that to a difference in its insects, birds, 
reptiles, and wild mammalia. On what causes the existence of 
the fly depends we do not know, but it is not improbable that some 
comparatively slight changes in the temperature or humidity of 
the air at a particular season, or the introduction of some enemy 
might lead to its extinction or banishment. The whole face of 
the country would then soon be changed : new species would 
