122 NATURAL HISTORY 



The question that you put with regard to those 

 genera of animals that are peculiar to America, viz. 

 how they came there, and whence? is too puzzling for 

 me to answer; and yet so obvious as often to have 

 struck me with wonder. If one looks into the writers 

 on that subject, little satisfaction is to be found. Inge- 

 nious men will readily advance plausible arguments to 

 support whatever theory they shall choose to maintain ; 

 but then the misfortune is, every one's hypothesis is 

 each as good as another's, since they are all founded 

 on conjecture. The late writers of this sort, in whom 

 may be seen all the arguments of those that have gone 

 before, as I remember, stock America from the western 

 coast of Africa, and the south of Europe ; and then 

 break down the isthmus that bridged over the Atlantic. 

 But this is making use of a violent piece of machinery : 

 it is a difficulty worthy of the interposition of a god ! 

 " Incredulus odi." 



TO THOMAS PENNANT, ESQUIRE. 

 THE 



NATURALISTS' SUMMER EVENING WALK. 



equidem credo, quia sit divinities illis 



Ingenium. VIRG. GEORG. 



WHEN day declining sheds a milder gleam, 

 What time the May -fly 6 haunts the pool or stream ; 

 When the still owl skims round the grassy mead, 

 What time the timorous hare limps forth to feed; 



6 The angler's May-fly, the Ephetncra vulgata, LINN, comes forth from 

 its aurelia state, and emerges out of the water, about six in the evening, 

 and dies about eleven at night, determining the date of its fly state in 

 about five or six hours. They usually begin to appear about the 4th of 

 June, and continue in succession for near a fortnight. See Swammerdam, 

 Derham, Scopoli, &c. 



