autonomic isolation. In such creatures as snails, it would 

 seem that the different forms of impregnational isolation 

 do not often arise before a considerable degree of diver- 

 gence has been reached; for usually the most nearly re- 

 lated races, or varieties of Achatinella are found on sepa- 

 rate groves of the same species of trees, in different parts 

 of the same valley; while the most closely related -roups 

 of varieties, that are classed as separate species, are 

 usually found in separate valleys; though sometimes in 

 the same valley, but living on different species of trees. 



In the case of plants, I have seen closely related species 

 growing beside each other, and in some such cases they 

 are found to flower at different months of the year, and in 

 other cases it would probably be found that they are held 

 apart by the prepotence of pollen of a given species on 

 stigmas of the same species. Tn some of these cases 

 where the present form of isolation is undoubtedly auto- 

 nomic, it may be impossible to say whether the new type 

 was not first developed in a few individuals, that for a 

 generation or two were partially isolated geographically 

 or locally in some sheltered nook. 



