440 PROCEEDINGS OF THE ACADEMY OF [1898. 



the same opportunities in this regard as the much larger females ; 

 and the fact that the males are in no case limited for space in which 

 to grow, as are the females, makes against the view that their small 

 size, as compared with the females, is due to pressure. On the whole, 

 it seems to me, that some factor, other than those mentioned, is in- 

 volved. 



Whatever the cause of the dwarfed form, it will be noted that in 

 Crepidula it operates by stopping cell-growth and division, and the 

 real causes of so fundamental a phenomenon are worthy of a more 

 extended study than I have, as yet, been able to devote to it. 



There is good evidence that these dwarfs are not a permanent va- 

 riety or race. In the first place there are no anatomical differences 

 between the two varieties save size only; secondly, the eggs, em- 

 bryos and larva? of the two cannot be distinguished • thirdly, there 

 is evidence that the dwarfs do not produce enough eggs to continue 

 the variety in its present numbers, for since the type and rate of de- 

 velopment are the same in the two varieties, it is probable that rel- 

 atively no more individuals will come to maturity in the one case 

 than in the other, and yet every giant female produces three times 

 as many ova as are produced by a dwarf; the relative number of 

 these two varieties remains practically constant from year to year, 

 and, therefore, I think it must follow that the ranks of the dwarfs 

 are continually recruited from the descendants of the giants. Both 

 live together on the same beach under about the same conditions of 

 food, temperature and water, the embryonic and larval development 

 of both forms are identical, and it seems probable that the adults of 

 both would be the same if one was not forced by the smaller quarters 

 which it inhabits to remain smaller than the other. But what is 

 still more conclusive is the following observation : A few specimens 

 were found which showed by the shape and character of their shells 

 that at one time they had been typical dwarfs; afterward, having 

 been detached, they obtained a new foothold on a larger surface, 

 and their shells increased in size, the new portions of the shell be- 

 coming shaped so as to fit the surface upon which they had found a 

 new home. In every such shell one can recognize both the dwarf 

 and the normal forms. The dwarfs are what they are by reason of 

 external conditions, and not because of inheritance ; they are, in 

 short, a physiological and not a morphological variety. In such a 

 case the shape and size of the body, as well as the number of cells 

 in the entire organism, are greatly modified by the direct action of 



