INTRODUCTION. 17 



reach an island like Ireland alive from the main- 

 land, and to colonise it successfully. 



Both slugs and their eggs are killed by a short 

 immersion in sea-water, as I have proved experi- 

 mentally. I have also subjected slugs, in the act of 

 crawling on twigs, to an artificial spray of sea-water. 

 This seemed to irritate their tender skins to such an 

 extent that they curled themselves up, released their 

 hold on the twig and let themselves drop to the 

 ground. If we supposed, therefore, that a slug had 

 successfully reached the sea, transported on a tree- 

 trunk, the moisture would tend to lure it forth from 

 its hiding-place under the bark, whilst the mere spray 

 would prove fatal to its existence. Those species of 

 snails and slugs which lead an underground existence, 

 rarely venturing above ground, such as Testacella and 

 Coecilianella, would have even less chance of being 

 accidentally carried to some distant island. 



The suggestion advanced by Darwin (p. 353), that 

 young snails just hatched might sometimes adhere to 

 the feet of birds roosting on the ground and thus be 

 transported, appears to me so extremely improbable 

 as to be scarcely worth serious consideration. Indeed, 

 as Darwin himself acknowledged later on, it does not 

 help us very much to suggest possible modes of 

 transport. What we require is direct evidence. How 

 far we are, however, from obtaining it, may be inferred 

 from Mr. Kew's remark (p. 119), that " we have 

 little or no actual evidence of precise modes of 

 dispersal even for short distances on land." 



