498 ZOOLOGY. 



seems originally to have been American, which entered Euro]*- 

 in tin- middle ages, and may now be found oven in the isles 



>!' < ><v,mica. 



In sonic 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 >eem 

 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, 1'uci floating en the surface of the water, supporting small 



rnstacea 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. 

 coa>ts along North America as high as Newfoundland, then 

 directs itself towards Iceland and Ireland, and red- 

 towards the Azores, often carries with it, even to the coast > 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 larva- of 

 insects, and may give attachment to the eggs of rnollusca, 



-. 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 digot 

 the , uv> thcv 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. Notwith- 

 standing these means of transport, and of other circumstances 

 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 studv 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. 



63*2. However this maybe, the obstacles to the geo- 

 graphical dissemination of species are sometimes altogether 

 mechanical, at other times physiological ; and amongst the 



