12 Our Food Mollusks 



upon several types of organisms, and each new creation 

 was made to conform more or less closely to one of them. 

 Having become a religious dogma, this idea was so firmly 

 fixed that it required a revolution in popular thought to 

 destroy it. The publication of Darwin's first great book 

 accomplished this end. 



The " Origin of Species " showed how natural forces 

 now in operation might produce new species from parent 

 forms. It presumed that the same forces had been 

 operating on organisms in the same way since the dawn 

 of life. According to this view, all living organisms have 

 a real relationship to each other, recent or remote. As a 

 rule, great differences in structure indicate distant, as 

 great similarities indicate close relationships. 



On account of fundamental similarities in develop- 

 ment, structure, and habit, which exist among oysters, 

 clams, scallops, mussels, and other members of the lamelli- 

 branch family, no naturalist now doubts that they 

 descended from some common ancestor, which, however, 

 must have lived in the very remote past, as man meas- 

 ures time. What this ancestral form was, is not posi- 

 tively known; but naturalists have agreed on what must 

 have been the general characters of many of its organs. 

 Why they should have a positive belief in regard to a 

 creature that no one ever saw, even in fossil form, is a 

 long story; but the reasons for it, if they were explained, 

 would probably be satisfactory to most minds. 



Among the very few bivalves here considered, it is 

 not easy to determine which, in its structure, conforms 

 most closely to the hypothetical ancestor. It is not the 

 black mussel, with its aborted foot and anterior adductor 

 muscle, and its sexual glands in the mantle folds. It is 

 not the scallop, in which much of the body is modified to 



