173 



to believe that, such is not the case ; and even 

 though it had been, there is nothing absurd in 

 the supposition, that stragglers may have been 

 occasionally wafted from such parts of the one 

 as were not very remote from the other, in a 

 kind of floating forest, which had been over- 

 thrown by the tempest, and borne by sweeping 

 rivers such as is still the case with the Missis- 

 sippi into the ocean. 



In general, the great exciting cause of migra- 

 tion among animals is the search of food. Thus, 

 among quadrupeds, wild deer move from one 

 part of the country to another in particular sea- 

 sons, as is exemplified in the rein-deer of Lap- 

 land and the northern parts of America, in order 

 that they may procure in perfection the necessaiy 

 supply of their aliment. So also the winged 

 denizens of the air fly from region to region, as 

 the insects on which they subsist are called in 

 succession by the march of the seasons into ex- 

 istence ; and it is with a similar object that the 

 fishes of the ocean perform their long periodical 

 voyages. 



If it is asked, what teaches these animals thus 

 to undertake far journey ings at particular times ; 

 I can only reply by demanding in return, what 

 teaches the orb on which we have our being 

 to revolve around the sun as a centre? what 

 teaches the heart of animals to pulsate, or the 

 sap to ascend, as regularly as spring returns, into 

 the minute vessels of the oak of the forest ? It 

 is to us inexplicable, otherwise than by referring 



