330 MASON : VARIATION IN THE SHELLS OF THE MOLLUSCA. 



tion of slight beneficial modifications of the individuals trans- 

 mitted from parent to offspring includes all these categories. 

 One very interesting example of this has been put on record by 

 Mr. E. S. Morse. As his account is interesting I will quote it. 

 "On a ledge in the harbour of Eastport (Maine), just east 

 of the town, a small variety of Buccinum xindatum occurs in 

 great profusion. At the time of collecting them the sexes were 

 pairing, and in every case (and hundreds Avere observed) the 

 male was much smaller, sometimes not exceeding half the length 

 of the female. It seemed impossible that the males could be 

 mature, and yet they were not only found paired, but an ex- 

 amination of the shell revealed the full number of whorls, and 

 other well-known characters indicated the fact that they were 

 full-grown though of diminutive size. 



"A glance at the condition of things at once revealed the 

 mystery of these dwarfed males. The ledge on which these 

 specimens were found is partly exposed at low tide, and is at all 

 times washed by impetuous currents, so that it is quite difficult 

 to land. 



"A study of the surface features of the ledge indicated the 

 force of the tidal currents. There were no loose fragments of 

 rock upon it, save those which were so tightly wedged in the 

 crevices of the ledge that they could not be worked out with 

 the hands. The specimens of Buccina in every case were found 

 hid away in nooks and concealed in the cracks and crevices 

 marking the ledge. It was clearly obvious that only the 

 smallest males could work their way into such constricted 

 quarters for the purpose of uniting with the female, and, that 

 the smaller males had the advantage over the larger males in 

 this respect there could be no question. The true state of the 

 case was instantly seen, and although hundreds of specimens 

 were collected with the object of determining whether in any 

 case a large male occurred, not a single exception was met with 

 in which the female was not being fertilized by a diminutive 

 male. 



J.C., vii., Jan. 1894. 



