32 HISTORY OF THE EUROPEAN FAUNA. 



what has been said on the subject generally. The two 

 species of snails, Helix pomatia and Cyclostoma elegans^ 

 both of which occur in England, and which I had occa- 

 sion to mention among those experimented on by 

 Darwin, were turned out in several suitable localities in 

 Ireland by Thompson, but failed to establish them- 

 selves. The former, according to Mr. Kew, was also 

 introduced into Scotland and Norway, whilst fifty or 

 sixty specimens were brought to Petersfield in Eng- 

 land, but none of these trials at acclimatisation 

 were successful. As among vertebrates, a large num- 

 ber of the so-called successful introductions rest upon 

 insufficient evidence. 



When we once more carefully review the evidence 

 as to the undoubted difficulty attendant on intentional 

 introduction of animals by human agency, placed as 

 they often were in most suitable localities, we must 

 feel that accidental introduction cannot play an im- 

 portant r61e in the making of the fauna of any country. 

 Especially is this the case with an island fauna. 

 Vertebrates are almost altogether excluded, and in- 

 vertebrates must arrive singly as a rule, often stranded 

 on an inhospitable and unsuitable shore. Their 

 chances of surviving a passage by sea, of finding 

 suitable food and shelter and a mate in order to pro- 

 create their species, appear to me infinitesimally small. 

 Yet there may be some such cases. However, I quite 

 agree with Mr. Andrew Murray a high authority 

 on geographical distribution that "colonisation or 

 'occasional dispersal is insufficient to account for the 



