Permanence and Evolution. 155 



springs up where closely resembling forms 

 already exist in the neighbourhood, which of 

 course suggests descent. 



It is now pretty generally known that simi- 

 larity of products between different areas 

 depends more on the presence or absence of 

 communication than on similarity of circum- 

 stances, and that organic types are diffused, 

 on the whole, in proportion to the means of 

 transport possessed by them ; that the number 

 of forms is small on oceanic islands ; that 

 barriers of any kind have an important in- 

 fluence on the facies of the flora or fauna of 

 any country. This is all true, but it has been 

 exaggerated. Difficulties have been slurred over. 

 Then, as I have often said, there will be always 

 found the difference between adventurous and 

 cautious minds. It is well if we remember that 

 conjectures are not science, however much they 

 may be prophecies of what may be science at 

 some future period. Sometimes the facts will 



