PREFACE. 



Hardly any branch of natural history has been so 

 neglected as that which treats of the various modes by 

 which the different classes of organisms have become 

 dispersed over the surface of the globe. Scattered 

 observations have indeed been made by many writers, 

 but Lyell and Darwin were the first to gather together 

 the existing evidence on the subject, ol* to test by 

 actual experiment the effects of exposure to salt water 

 on the vitality of seeds and land-shells. Owing to this 

 neglect the idea has arisen that seas of very moderate 

 width serve as complete barriers to the dispersal of 

 most living things ; and it has been thought necessary 

 to postulate great and often repeated geographical 

 mutations, and even to bridge across the widest and 

 deepest oceans, in order to account for the actual dis- 

 tribution of mammals or reptiles, of plants, insects, or 

 terrestrial mollusca. 



It was Darwin who first taught us that these assump- 

 tions of vast and repeated changes in the distribution 

 of sea and land were at once inadmissible and un- 

 necessary. By his original and masterly investigation 



