GEOGRAPHICAL DISTEIBUTION OF ANIMALS. 543 



In some instances animals have been able to burst natural 

 barriers seemingly insurmountable, and to spread themselves 

 over a more or less considerable portion of the surface of the 

 globe by means of circumstances which at first sight seem 

 extremely unimportant, such as the movement of a fragment 

 of ice, or of a morsel of wood swept along by currents to dis- 

 tances often very considerable : thus nothing is more common 

 than to meet at sea, at a distance of hundreds of leagues from 

 all land, fuci floating on the surface of the water, supporting 

 small Crustacea incapable by themselves of removing by 

 swimming to any great distance from the coasts where they 

 were produced. The great maritime current which, leaving 

 the Gulf of Mexico, coasts along North America as high as 

 Newfoundland, then directs itself towards Iceland and Ireland, 

 and redescends towards the Azores, often carries with it, even 

 to the coasts of Europe, trunks of trees, which the Mississippi 

 has torn away from parts the most remote of the New World, 

 and carried to the sea. Now these timbers are often bored 

 by the larvse of insects, and may give attachment to the eggs 

 of mollusca, insects, &c. Finally, even birds contribute to 

 the dispersion of living beings over the surface of the globe, 

 and that in a most singular manner ; these animals often do 

 not digest the eggs they swallow, and, discharging them at 

 considerable distances from the place where they had found 

 them, transport to a distance the germs of a race unknown 

 to that time in the countries where they have been deposited. 

 Notwithstanding these means of transport, and of other cir- 

 cumstances equally calculated to favour the dissemination of 

 species, there are really very few animals cosmopolitan, and 

 most of these beings are cantoned in regions sufficiently limited. 

 Moreover, we comprehend why it should be so in studying 

 the circumstances which may oppose their progress. But 

 this study is far from furnishing us with a sufficient explana- 

 tion of the limited circumscription of a species, and it is often 

 impossible for us to divine why certain animals remain con- 

 fined to a locality when there seems to be nothing opposed to 

 their propagation in neighbouring districts. 



632. However this may be, the obstacles to the geo- 

 graphical dissemination of species are sometimes altogether 

 mechanical, at other times physiological; and amongst the 

 first we may mention seas and high chains of mountains. 

 For terrestrial animals, in fact, seas of a certain extent form in 

 general an insurmountable barrier ; and we see that, all things 



