52 On the Vegetable Colomsatlon of the British Islands. 



of the Feroe group ; on the same day they were carried to 

 ShetlandSjthe Orkneys, and collected from the decks of vessels 

 sailing between England and Ireland. 



There is still another mode of transportation peculiar to 

 the countries we are considering, and which does not ap- 

 pear to have been hitherto taken into account ; I mean mi- 

 gratory birds. Every spring, millions of sea-birds leave the 

 coasts of Spain, France, and England, and migrate to the 

 north, in order to lay and hatch their eggs in the steep and 

 deserted shores of Shetland, Feroe, and Iceland : the follow- 

 ing autumn they return to Europe with their brood. These 

 birds transport rapidly from one island to another the seeds 

 of vegetables preserved in their gizzards, or attached to their 

 feathers. They thus become the principal agent of the Ameri- 

 can migration, for it is exactly towards the end of the sum- 

 mer of these regions, at the time when seeds are ripe, that 

 they return towards the south. In this journey, the islands 

 of which we have been speaking serve them as a kind of pro- 

 visional magazines or resting-points, and they sow there the 

 seeds they have transported through the air. 



When we reflect that all these causes united have acted in- 

 cessantly since the beginning of the present epoch, that is to 

 say, for thousands of years, we cannot question the principal 

 effects of an action so prolonged. Accordingly, before going 

 back in the series of geological ages to explain the distribu- 

 tion of organised beings on the surface of the globe, we must 

 demonstrate the insufficiency of existing causes. This plan is 

 applicable to all the problems of geology. Formerly, in the 

 theological period of this science, the most extravagant ima- 

 ginations were boldly indulged. According to the necessi- 

 ties of the case, sudden revolutions were supposed, or prodi- 

 gious overthrows, colossal forces, unknown agents, and ima- 

 ginary causes. Now, reflecting minds first seek the reason 

 of geological facts in natural forces, acting within the limits 

 of the power which they develop to our own view, and do 

 not enter the field of conjecture till they have exhausted that 

 of reality. — Bib. Universelle, June 1848.* 



* After the above was in types, we received, but too late for insertion, from 

 M. Martins, a more detailed memoir on vegetable colonisation. — Edit. 



