454 On the Lajid and Fresh Water 



But if we call in the aid of geology, we shall find that, in all pro- 

 bability, this great mountain barrier is of later date than the 

 fauna and flora existing around it. It should be stated too that 

 the fresh water Pulmonifera are remarkable for their world-wide 

 distribution. 



The laws which affect the geographical distribution of plants 

 a,nd animals on the surface of our planet, are creating much inte- 

 rest just now in the minds of scientific men. Analogy it has 

 been said favours the supposition that each species whether ani- 

 mal or vegetable was originally formed in some particular locality, 

 whence it spread itself gradually over a certain area ; rather than 

 that the earth was at once, by the fiat of the Almighty, peopled 

 as we at present behold it. The majority of our best naturalists are 

 inclined to accept the theory that every species has originated 

 from a common centre, and that numerous such centres were sit- 

 uated in different parts of the world, each centre being the seat 

 of a particular number of species. In accordance with this view, 

 Mr. Woodward, in his admirable treatise on recent and fossil 

 shells, has mapped out the whole globe into moUuscan provinces, 

 each of which he supposes to possess a certain number of shells 

 peculiar to it, and to be characterized by definite groups of this 

 class of animals. Prof. Schouw, of Berlin, has carried out the 

 same idea in the vegetable kingdom ; but the views of these two 

 gentlemen do not exactly correspond. Mr. Woodward divides the 

 eastern part of North America into two regions, characterized, ac- 

 cording to his views, by a peculiar assemblage of land and fresh water 

 shells. One of these he calls the Canadian region, which includes 

 the whole of Upper and Lower Canada ; — and the other the Atlan- 

 tic region, which comprises all the United States east of the Missis- 

 sipi valley. In Europe generally, even at the present date, but little 

 is known respecting the natural history of Canada. Hence Mr. 

 Woodward's data were hardly suflScient to enable him to generalize 

 with much confidence. He remarks, "the country drained by 

 the great lakes, and the river St. Lawrence possesses very few pe- 

 culiar shells, and those mostly of fresh water genera. It is 

 chiefly remarkable for the presence of a few European spe- 

 cies, which strengthen the evidence of a landway across the 

 Atlantic having remained till after the epoch of the existing ani- 

 mals and plants." 



This landway I propose to say a few words about presently. 

 And here, it may be observed, that of all the land snails which 

 are common to both sides of the Atlantic, very few can be proved 



