INSTITUTE OF SCIENCE, PHILADELPHIA. 355 



also liable to be exerted on the fulcrum or septum which may tend to modify 

 it as compared with the septum of an individual seated on a broad or flat 

 pedestal. These characters, once assumed, might be more or less fortified by 

 natural selection, but it is also obvious that they may be induced and main- 

 tained directly by environmental influences, and in my opinion many charac- 

 ters, especially in littoral mollusks, are directly so maintained and originated ; 

 the influence of selection being of comparatively small importance, so far as 

 they are concerned. 



That this is the case in the genus Crepidula, I have little doubt. The 

 opinion is fortified by the fact that in all countries where Crepidulae flourish, 

 and the circumstances admit it, we have certain forms of shell developed in 

 connection with a certain situs. Beside a large species, in our case C.fonii- 

 cata, suited for oyster-beds, large bivalves, flat stones and similar situations, 

 we have a flat form, C. plana (or unguiformis^, found in dark, concave 

 places, especially the interior of dead gastropod shells. I have received C. 

 plana from Texas, bearing in fine yellow-brown lines the distinctive pattern of 

 fornicata's ramose color-markings. When a C. plana gets out of place and 

 develops on a convex base such as the outside of a gastropod shell, the 

 beak will almost always show a reminiscent touch of brown color, indicating 

 that the pressure of the environment which produced in the parent whiteness 

 and depression being removed, the hereditary characters of the ancestry 

 begin, especially in the young, to reassert themselves. I suspect that if C. 

 plana were reared for several generations in an aquarium, where it was obliged 

 to live on a wide convex surface, it would rapidly lose its acquired (and sup- 

 posedly characteristic) whiteness and peculiar form. I am aware that it has 

 been claimed that the dentition and other features in the section lanaciis are 

 different from those of the true Crepidula, but (apart from the modification in 

 these due to environment) the amount and importance of these differences 

 have never been subjected to a sufficiently thorough test to indicate their 

 value. 



Again, in nearly all coasts we have a form of Crepidula which is found 

 seated on a convex and limited area, such as the shell of another mollusk or 

 the surface of small pebbles. On our own coast this form is represented by 

 C. convexa, on the California coast by C. adunca, etc. The differences these 

 shells exhibit by reason of heredity, selection and environmental pressure 

 may have become sufficiently fixed to be regarded as of specific value. The 

 point to which I desire to call attention is that, primarily, the differences are 

 not due to variation and selection, but to the immediate and effective impact 

 of the environmental forces. As we go back in the geological column and 

 examine large numbers of specimens, we find that intermediate individuals and 

 graduated differences are far more common than in the recent representatives 

 of the same type. This is what theory would lead us to expect, and it is sat- 

 isfactory to recognize in this way the truth of our notions as to the action of 



