The geographical distribution of animals, with a study of the relations ...

44 DISTRIBUTION OF ANIMALS. [part i.different vegetation and a considerably modified fauna to occupythe country.Organic Changes as affecting Distribution.—We have now brieflytouched on some of the direct effects of changes in physicalgeography, climate, and vegetation, on the distribution of animals; but the indirect effects of such changes are probably ofquite equal, if not of greater importance. Every changebecomes the centre of an ever-widening circle of effects. Thedifferent members of the organic world are so bound together bycomplex relations, that any one change generally involvesnumerous other changes, often of the most unexpected kind.We know comparatively little of the way in which one animalor plant is bound up with others, but we know enough to assureus that groups the most apparently disconnected are oftendependent on each other. We know, for example, that theintroduction of goats into St. Helena utterly destroyed a wholeflora of forest trees ; and with them all the insects, mollusca, andperhaps birds directly or indirectly dependent on them. Swine,which ran wild in Mauritius, exterminated the Dodo. The sameanimals are known to be the greatest enemies of venomousserpents. Cattle will, in many districts, wholly prevent thegrowth of trees ;and with the trees the numerous insects dependenton those trees, and the birds which fed upon the insects,must disappear, as well as the small mammalia which feed onthe fruits, seeds, leaves, or roots. Insects again have the mostwonderful influence on the "range of mammalia.In Paraguay acertain species of fly abounds which destroys new-born cattleand horses ;and thus neither of these animals have run wild inthat country, although they abound both north and southof it.This inevitably leads to a great difference in the vegetation ofParaguay, and through that toa difference in its insects, birds,reptiles, and wild mammalia. On what causes the existence ofthe fly depends we do not know, but it is not improbable that somecomparatively slight changes in the temperature or humidity ofthe air at a particular season, or the introduction of some enemymight lead to its extinction or banishment. The whole face ofthe country would then soon be changed : new species would