1897.] NEW YORK ACADEMY OF SCIENCES. 81 



Graf* concludes that we have here a ridged variety of Crepidula 

 which in time niaj- become established as a species. From the 

 evidence yielded by the Placuanomia, the fossil shells and the 

 Myochama^ I would, however, attach less importance to the phe- 

 nomenon. A character is here initiated and sustained by a con- 

 tinuous extraneous stimulus, and, as shown by the Myochama, 

 it vanishes the moment the stimulus falls below a certain inten- 

 sit3\f Moreover, all the forms studied appear to attach them- 

 selves indifferently to surfaces of diverse and varied patterns, 

 and consequently the stimulus is too varied and changing for 

 an}"^ particular pattern to become impressed upon the germ- 

 plasm and reappear in the offspring. This could only occur in 

 case the habit was acquired of always fixing upon a surface of 

 similar pattern.^ 



It is, however, not unlikely, as Mr. Yan Ingen suggests, that 

 this adaptation may in some cases be of advantage to the shell 

 by enabling it to cling more tightly to the attached surface. The 

 fact that some individuals readily assume the surface markings, 

 while others do not, would seem to indicate that the plasticity 

 of the mantle varies. This variability then furnishes a possible 

 field for the operation of natural selection. The individuals 

 fa-v^red, however, would certainly be those with sensitive and 

 plastic mantles, readily adaptable to any character of surface. 

 On the other hand those shells which tend to reproduce a defi- 

 nite pattern would be at a disadvantage in adapting themselves 

 to the different hosts available for attachment. 



Before, even an approximate decision can be reached in 



respect to these theoretical questions a detailed comparative 



.study should be made of all the available facts, not forgetting 



the late embryology of these attached shells. Such a careful 



study the phenomenon under consideration seems to well merit. 



In conclusion, I wish to express my obligation to Professor 

 Whitfield and Mr. Gratacap for kindly allowing me access to the 

 collection of shells in the American Museum of Natural His- 

 tory, New York, and permitting me to make a drawing of the 

 3Iyochamh. I am also indebted to my friend, Mr. Yan Ingen, 

 for suggestions as to literature bearing on this subject. 



ZooiiOGicAL Department, Columbia University. 



* /. c. p. 69. 



tThe' fact that the faintest possible stimulus is sufficient to produce an effect 

 upon Placuamonia, while a more powerful one is recjuired in Myochama, may be 

 attributed to the fact that the former is relatively a smooth shell, while the latter is 

 ridged, since it is readily conceivable that the folds of the attached surface would need 

 be of considerable prominence to reverse and suppress the tendency to ridges in the 

 Myochama. . „ . , , 



J The susceptibilitv of the germ-plasm to extraneous influences is here merely 

 assumed for argument's sake. 



Transactions N. Y. Acad. Sci., Vol. XVI., Sig. G, March 26, 1897. 



