1 898.] NATURAL SCIENCES OF PHILADELPHIA. 439 



snail (Limncea gtagnaHx) remained small when grown in a small 

 quantity of water, while the larger the quantity of water, up to 4,000 

 or 5,000 cc, the larger the snails reared in it. As the result of nu- 

 merous experiments, Semper concluded that this difference in size 

 was not due to differences in the quantity of food, inorganic salts or 

 oxygen obtainable, and he suggested that some unknown substance 

 must be present in the water which acts as a stimulus to growth 

 without actually contributing to it. 



More recently, DeVarigny 7 has repeated these observations, and 

 concludes, as the result of several experiments, that the relative vol- 

 ume of the water, in which the snails are grown, is much less import- 

 ant than the relative amount of surface exposed. He holds that the 

 larger the surface the more exercise the animals are able to take, 

 and, therefore, the larger they become. His results show that Sem- 

 per's conclusions are untenable, but they by no means establish his 

 own. It is certainly not generally true, as he holds, that physiolog- 

 ical or mechanical impedimenta to movement result in dwarfing. 

 The larger forms of C. plana are as immovably fixed as the dwarfs ; 

 in this case, therefore, movement can have nothing to do with body- 

 size. 



In Crepidula, the dwarfed form is unquestionably correlated with 

 the smaller size of the shell in which it has found lodgement. It is 

 possible that the diminished size is due to diminished supply of food 

 or oxygen ; however, the following observation is opposed to this 

 view : I have never found more than one mature female in a shell 

 inhabited by the small hermit, whereas, from four to eight very 

 large individuals may be found in the shell of a large hermit ; under 

 these circumstances, it seems very improbable that the difference in 

 size is due to differences in the amount of food or oxygen obtainable. 

 The most natural interpretation is that the dwarfing is due to pres- 

 sure which limits growth in various directions ; though it must be 

 confessed that the shell of the dwarf remains thin and delicate, 

 whereas, the shells of the common form which are limited in growth 

 by surrounding hard parts grow thick and have a distorted appear- 

 ance. The fact, also, that the males of all these species, and espe- 

 cially of G plana, remain very much smaller than the females (as is 

 pointed out in Section III), speaks against the view that the smaller 

 size is due to a diminution of food or oxygen, since the males have 



7 DeVarigny, Experimental Evolution, 1891. 



