SOCIAL "CROWNED HEADS." 125 



mistaken the one for the other. Further, in all 

 three there is so great a likeness in general as to 

 suggest the idea of a common origin, and to make 

 the inquiry into their affinities and into the cause 

 or causes of their divergence a subject of special 

 interest. This is one of the chief charms in the 

 study of Nature since Darwin furnished the key to 

 the inquiry; and when we find those similarities 

 accompanied with yet specific differences enough 

 to have led former naturalists to set them (as in 

 this case) in different families, one naturally looks 

 around for some explanation sufficiently feasible 

 to account for those differences. 



What special circumstances, for example, could 

 have led to the formation of the free and revolving 

 spherical condition which we find in Conochilus ? 

 Presuming it to have descended from the same 

 ancestor as Lacinularia and Megalotrocha how has 

 it acquired a habit and motion so different from 

 theirs ? Or the question may be reversed, and 

 by assuming the latter to have descended from 

 Conochilus, or even both from another form 

 not present to our inquiry, still the question 

 as to the origin of the differences, and the in- 



