SELECTION NOT ALWAYS THE CAUSE. 5 



that they possess the power of discriminating colors, we must seek 

 some other explanation. If the colors were of a protective character 

 we might surmise that they had been developed through exposure 

 for many generations to sharp-sighted enemies; but in the case of 

 many of the species, their white and green tints striped with black 

 make them conspicuous objects against the brown trunks of the trees 

 on which they are constantly found. Neither are their colors for 

 warning; for these snails are not repulsive to flesh-eating birds. It 

 is, therefore, hard to avoid the thought that these striking colors are 

 of no service in protecting the life, either of the individual or of the 

 species. Again, certain birds of prey are the only aboriginal crea- 

 tures that could be suspected of feeding on these snails; and as the 

 birds have a wide range, the immense diversity of color in the snail 

 shells of one island would remain unexplained. 



There is, also, another character in which species of some of these 

 genera often differ from each other, in regard to which natural selec- 

 tion has never been shown to be the controlling factor. I refer to the 

 character of the coil of the shell, which may be dextral in one species, 

 sinistral in another, and either dextral or sinistralin a third. As long 

 as it is impossible to give any reason why a species would not be 

 equally successful if every individual possessed the reverse form from 

 that it now has, it is unreasonable that we should attribute the pres- 

 ent form to the influence of natural selection. 



The theory that there is great advantage for the species in having 

 all the individuals coiled in the same way, if proved, would in no way 

 explain why certain species are always dextral and certain others are 

 always sinistral, while some well-established species present large 

 masses of individuals of each form. I think it will some day be 

 shown that snails of opposite forms, though of the same race, are in- 

 capable of mating with each other, and we already know that each 

 individual is both male and female. If, then, an unusual sport should 

 produce, in the same family, or on the same tree, two individuals of 

 the reverse form from their parents, we should have a completely 

 segregated variety established in the original home, while exposed to 

 the same environment and using it in the same way, and its new char- 

 acter conferring no benefit. 



Third. Because in the case of certain divergent species of Achatinella, 

 occupying isolated valleys presenting the same vegetation, the diversity of 

 selection to which they are subjected through different habits of feeding is 

 directly preserved by the isolation which prevents the peculiar habits from 

 being broken down by free crossing. The habits of feeding are not with- 

 out variation even in the original valley. When, therefore, one or two 



