MOLLUSCS, OB SHELL-PISH. 237 



their differences are directly associated with the differences 

 in their surroundings. The latter, by virtue of its hairy 

 coat, can live among mud, which the former cannot. It, 

 on the other hand, from its superior agility, can probably 

 escape enemies which the other could not, and is therefore 

 enabled to live in more exposed places. If hybrids between 

 the two were to occur they would probably be well adapted 

 for the habitat of neither, and so would tend to be elimi- 

 nated. In other words, marked and permanent differences 

 of environment tend to produce marked and permanent 

 differences in species. 



If we return to the periwinkles we find that there is no 

 such marked difference in the environment in this case. 

 They are tolerably active animals, and therefore, though 

 each species may theoretically have its own zone, its mem- 

 bers seem to wander freely into the zones of other species. 

 This must have two consequences. In the first place, 

 divergence will be probably checked by constant inter- 

 breeding; secondly, if the adults wander freely, their 

 adaptation to any particular locality cannot be very exact, 

 and there is no reason to believe that hybrids will be more 

 likely to be eliminated than pure forms. That is, the 

 species should not be well defined. Now this is what 

 actually occurs; there are a great number of periwinkles, 

 and in many cases it is almost impossible to distinguish 

 between species and varieties. There can, I think, be no 

 doubt that this is due to the continuity of the environment. 

 It naturally, however, makes the identification of species 

 very difficult, and the distinction of species a fruitful source 

 of controversy. A modern will no doubt say their distinc- 

 tion is a matter of no importance; but if the attempt 

 makes clear the meaning of variation, it is not without its 

 usefulness. 



Let us first answer the question, What is a periwinkle? 

 All the periwinkles have solid top-shaped shells, with a 

 short spire and an entire mouth. The surface of the shell 

 is sometimes smooth, sometimes spirally grooved. The 

 mouth is nearly circular, and has a sharp-edged outer lip, 

 while the other or columellar lip is expanded. The oper- 

 culum is pear-shaped, horny, spirally coiled, with its centre, 

 or nucleus, laterally placed. In the living animal the head 



