150 



when seventeen miles off Cape Corrientes, I had a net 

 overboard to catch pelagic animals. Upon drawing it 

 up, to my surprise I found a considerable number of 

 beetles in it, and, although in the open sea, they did not 

 appear much injured by the salt water. I lost some of 

 the specimens ; but those which I preserved belonged to 

 the genera Colymbetes, Hydroporus, Hydrobius, Nota- 

 phus, Cynucus, Adimonia, and Scarabaeus. At first I 

 thought that these insects had been blown from the 

 shore ; but upon reflecting that, out of the eight species, 

 four were aquatic (and two partly so) in their habits, it 

 appeared to me most probable that they were floated 

 into the sea by a small stream which drains a lake near 

 Cape Corrientes. On any supposition, it is an inter- 

 esting circumstance to find live insects swimming in the 

 open ocean seventeen miles from the nearest point of 

 land*." 



Accidental means of dissemination, such as those to 

 vyhich I have just alluded, and others to which we might 

 appeal, will generally account, and with much presump- 

 tive truth, for the many exceptional cases which present 

 themselves, during our investigation into the effects of 

 natural barriers, as visible in the distribution of the 

 Annulose races, on the earth's surface. I say " excep- 

 tional cases," because any one who has laboured practi- 

 cally in mountain tracts cannot have failed to recog- 

 nize the marked difference which is often displayed by 

 the insect population on opposite sides of some alpine 

 * Journal of Researches, p. 159. 



