MASON : VARIATION IN THE SHELLS OF THE MOLLUSCA. 33 1 



" The constrained position in which these were found pre- 

 cluded the possibility of a large male, with his cumbrous shell, 

 getting sufficiently close to the female in her narrow quarters. 

 The smaller males having this advantage, have from generation 

 to generation perpetuated their dwarf characters. It would 

 seem from these facts that natural selection has worked in an 

 unusual way in producing secondary sexual characters, rarely, 

 if ever, seen in the moUusca. 



"Both males and females presented a wide range of varia- 

 tion in the character of the shell, some of them showing very 

 distinctly the oblique folds so characteristic of the species, 

 while in others the folds were scarcely visible. The shell of the 

 male is smootlier than that of the female, and is also more 

 slender and more delicate." 



Certain genera seem to be especially prone to evolutionary 

 influences, or, in other words, the process of evolution has not 

 yet proceeded far enough to produce stable species. This is 

 strikingly exemplified in plants among the brambles, roses, 

 hawkweeds, and willows. Examples among the moUusca are 

 found in the freshwater Naiad genera Unio and Anodon, and in 

 the Pulinonate genus Succinea. 



In plants it is easy to test the effects of change of environ- 

 ment, but for obvious reasons this is impossible among the 

 mollusca. Variation, then, may be simply defined as divergence 

 from the type. 



But the time at my disposal will not allow me to enter 

 into any systematic examination of Professor Brandt's sub- 

 divisions. The place which the moUusk fills in the kingdom 

 of nature in itself limits the possible number of variations. On 

 the one hand the moUusk has reached a plane of development 

 in which it has climbed above the excessive multipHcation of 

 similar organs, as in the Annelids, and the stage in which a large 

 number of quasi-independent individuals live in a common 

 Zooecium, as in the Molluscoid Bryozoa. On the other hand, 



