542 ZOOLOGY. 



whilst the wild duck is found everywhere, from Lapland to 

 the Cape of Good Hope, and from the United States of 

 America to China and Japan. 



The circumstances which favour the dissemination of spe- 

 cies are of two kinds. The first is connected or dependent on 

 the nature of the animal itself; the second, with causes foreign 

 to it. In the number of the first, the development of the 

 locomotive power holds an important place. All things being 

 equal, the species which live fixed to the soil, or which possess 

 but imperfect instruments for locomotion, occupy but a re- 

 stricted portion of the surface of the globe, compared with 

 species whose movements of translation are rapid and ener- 

 getic. Thus, amongst terrestrial animals, birds offer us most 

 examples of cosmopolitan species ; and amongst the aquatic 

 animals, cetacea and fishes. Reptiles, on the contrary, are 

 generally cantoned on narrow limits ; and the same may be 

 said of most of the molluscs and of the Crustacea. The in- 

 stinct which leads certain animals periodically to change their 

 climate, contributes also to cause the dissemination of species ; 

 and this instinct, as we have already seen, exists in a great 

 number of these beings. Amongst the circumstances foreign 

 to the animal, and in some measure accidental, concurring to 

 bring about the same result, the influence of man may be 

 placed foremost ; and to give of this an exact idea, it will be 

 sufficient to mention a few species. The horse originally 

 belonged to the steppes of Central Asia ; and at the epoch of 

 the discovery of America, there did not exist in the New 

 World an individual of the species. The Spaniards trans- 

 ported the horse with them at an epoch which does not 

 ascend beyond three ages ; and in our day, not only the in- 

 habitants of this vast continent, from Hudson's Bay to Terra 

 del Fuego, possess horses in abundance, but these animals 

 have even recovered their wild condition, and are found in 

 troops almost innumerable. It is the same with our domestic 

 ox. Carried from the Old to the New World, they have in- 

 creased to such an extent, that in some parts of South America 

 they are hunted solely with a view to obtain the hides for the 

 manufacture of leather. The dog also has been everywhere 

 the companion of man ; and we may add to the number of 

 animals become cosmopolitan in our time, the rat, which 

 seems originally to have been American, which entered 

 Europe in the Middle Ages, and may now be found even in 

 the isles of Oceanica. 



