VEGETABLE COLONIZATION. 237 



neys,and were observed on the decks of vessels sailing between England 

 and Ireland. 



There is another mode of transportation peculiarly incident to the 

 countries we are considering, and which I do not remember to have 

 seen noticed. I allude to birds of passage. Every spring millions of 

 maritime birds leave the coasts of Spain, France, and England, and 

 emigrate northward to lay and hatch their eggs on the solitary cliffs of 

 Shetland, Faroe, and Iceland. The autumn following they return to 

 Europe with their broods. These birds transport rapidly from isle 

 to isle the seeds of plants preserved in their crops or attached to 

 their feathers. They become in like manner a principal instru- 

 mentality in the American migration, for it is precisely at the close of 

 the summer of those regions, when the seeds are at maturity, that 

 they return towards the south. In these voyages the islands we speak 

 of serve for resting places, where they deposite the seeds which they 

 have transported through the air. 



When we reflect that these united causes have been acting inces- 

 santly from the commencement of our era, that is, for thousands of 

 years, it is impossible to cast a doubt, on what must have been the 

 prominent effect of such prolonged agencies. Before ascending, then, 

 through the series of geologic ages to explain the distribution of or- 

 ganized beings over the surface of the earth, it would seem obvious that 

 the insufficiency of existing causes should be first demonstrated. And 

 this method is applicable to all the problems of geology. Heretofore, 

 during what might be called the theologic period of that science, it was 

 too much the custom to launch into the most extravagant supposi- 

 tions. According to the exigencies of the case, sudden revolutions,, 

 overwhelming catastrophes, colossal forces, unknown agents, and fan- 

 tastic causes were pressed into the service. At present, inc[uiry, having 

 become more sedate, seeks first the reason of geologic facts in the 

 powers of nature acting within the limits which they observe according 

 to our own experience, and enters not the field of hypothesis without 

 having first exhausted that of reality.* 



ON THE CAUSES WHICH LIMIT VEGETABLE SPECIES TO- 

 WARDS THE NORTH, IN EUROPE AND SIMILAR REGIONS. 



BY MR. ADOLPHE DE CANDOLLB. 



[Read before the Academy of Sciences of Paris, December 13, 1847, and translated for the 



Smithsonian Institution.] 



It was said by Linna3us a hundred years ago that ''all real know- 

 ledge is founded on specific knowledge." And, in efi'ect, in all the 

 branches of natural history, a thorough knowledge of species is the 



*^ The causes which M. Martins has assigned above have doubtless had their influence in re' 

 gard to the flora of the islands in question, but they are not suflicient to explain the peculiarities 

 of the fauna of those regions, and do not, therefore, essentially militate agamst the hypothesis 

 of Messrs. Forbes and Watson— Traxs/u^or. 



