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 



