6 INTRODUCTION. 



individuals with a special habit of feeding are transported to a new 

 valley, it is not at all strange that they choose to pass by some plants 

 used by many varieties of the species in the original home. More- 

 over, their descendants may never regain the power to feed on as many 

 kinds of plants; or, if on as many, probably not on the same kinds. 

 Still further, when a distant valley has been reached, after a number 

 of such transfers, each transfer being followed by a long history of 

 habit building, without the influence of crossing with individuals of 

 the original stock, is it at all strange that the habits have become 

 widely divergent, and that they are the cause of divergent selection 

 tending to establish the divergent habits in a more fixed form? 



In such a case as the one just described the new habit and the 

 diversity of selection, with the diversity in the direct influence of the 

 environment, can not be ascribed to any advantage over the old form 

 resulting from the new habit ; for competition with the old form has 

 ceased for countless generations. Moreover, if the later history of 

 the newer species has been in a new and unoccupied district, compe- 

 tition with all allied forms has ceased; and the new habit is simply 

 one of several forms of using the environment that are open to a new 

 colony, unaffected by constant crossing with the old stock. 



III. May not the Prevention op Free Crossing be an Expi^anation ? 



It will probably have already occurred to many of my readers that 

 the wonderful limitation in the areas of distribution occupied by the 

 separate species of Hawaiian snails is in some way connected with 

 lack of powers and opportunities for migration ; and now as we reflect 

 that the same lack of migration would immeasurably increase the 

 isolating effect of some rare occurrence by which a single individual 

 is carried a mile or two beyond the home of the species, into a region 

 of abundant food, the question naturally arises whether the isolation, 

 which prevents all chance of crossing with the original stock, does not 

 open the way for new habits, for new forms of selection, and accord- 

 ingly for the transformation of the new colony into a new variety, 

 and finally into a new species. 



We have here reached the idea of freedom from crossing with the 

 old stock, or isolation, in the broader meaning which has been given 

 to the term by writers on evolution since the days of Darwin. I be- 

 lieve that no process of natural selection, or of sexual selection, or of 

 any other form of selection, can transform one species into two or 

 more species without the prevention of free crossing between the 

 branches that are thus transformed. Isolation is, I beUeve, an essen- 



