106 GEOGRAPHICAL DISTRIBUTION 



Among the circumstances foreign to the animal, and in a mea- 

 sure accidental, we place first the influence of man ; and to illus- 

 trate this point, a few examples will suffice. The horse is origin- 

 ally from the steppes of Central Asia, and, at the time of the 

 discovery of America, no animal of this species existed in the 

 New World ; the Spaniards carried it with them there not mors 

 than three centuries back, and now, not only do the inhahitants 

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

 possess horses in abundance, but these animals have become 

 wild, and are found in almost countless herds. The same is true 

 of the domestic ox : carried 1'rom the Old to the New World, 

 they have multiplied there to such an extent that in some parts 

 of South America they are actively hunted for their hides only, 

 for the manufacture of leather. The dog has been everywhere 

 the companion of man, arid we could instance a great many ani- 

 mals that have become cosmopolite by following us; the rat, 

 which appears to be originally from America, overran Europe in 

 the middle ages, and is now met with even on the islands of 

 Ocea'nica. 



In some cases, animals have been able to break through natural 

 barriers, seemingly insurmountable, and spread themselves over 

 a more or less considerable portion of the surface of the globe, 

 by the assistance of circumstances whose importance at first 

 sight seems very trifling, such as the movement of a fragment of 

 ice or wood, often carried to considerable distances by currents: 

 nothing is more common than to meet at sea, hundreds of miles 

 from land, fucus floating on the surface of the water and serving 

 as a resting-place for small crusta'ceans incapable of transport- 

 ing themselves, by swimming 1 , far from the shores where they 

 were born. The great maritime current, the gulf-stream, com- 

 mencing in the gulf of Mexico, coasts North America to New- 

 foundland, then directs its course to Iceland, Ireland, and returns 

 towards the Azores, often bearing to the coasts of Europe, trunks 

 of trees which were conveyed by the waters of the Mississippi, 

 from the most interior parts of the New World, to the sea ; it 

 frequently happens that these masses of wood are perforated by 

 the larvaB of insects, and they may afford attachment to the eggs 

 of mollusks, and of fishes, &c. Finally, even birds contribute 

 to the dispersion of living creatures over the surface of the globe, 

 and that too in a most singular manner : frequently they do not 

 digest the eggs they swallow, but, evacuating them at places far 

 from where they were picked up, carry to great distances the 

 germs of races unknown till then in the countries where they 

 were deposited. 



Notwithstanding all these means of transportation and other 

 circumstances favouring the dissemination of species, there are 



