HARMFUL EFFECTS OF CROWDING UPON GROWTH 109 



volume of the water .... the attainment of full size within a definite 

 period would only be possible if the volume of water were so great 

 that the Lymnaea could at all times absorb this unknown stimulant 

 from the water." This hypothesis, in some form or other, has been 

 proposed, apparently independently, by a number of workers since 

 Semper's time. 



Semper seems to have been certain of the evolutionary significance 

 of the Hmitation of growth by volume. He found it impossible to 

 obtain full-sized individuals from snails stunted during the first year 

 of their lives; and if the causes checking growth were repeated regu- 

 larly through the succeeding generations, he felt that a dwarfed race 

 must arise. Whitefield (1882) came to the same conclusion, using 

 Lymnaea megasoma from Vermont. Whitefield continued the crowd- 

 ing for four successive generations, during which time the snails be- 

 came successively smaller and more slender, so that an experienced 

 conchologist did not recognize their relation to the shells of the 

 parent stock. 



Yung (1878, 1885) concluded from his experience in raising tad- 

 poles in containers of various sizes and shapes that dwarfing is due to 

 a lack of aeration. De Varigny (1894) took up the problem with 

 Lymnaea again and in general obtained the same sort of results 

 reported by both Semper and Yung. A snail kept in a liter of water 

 with a surface of 257 sq. cm. for 5 months was nearly twice the 

 length of one kept in the same volume of water but with a surface 

 area of 3.14 sq. cm. In order to facilitate the analysis, De Varigny 

 suspended a glass tube 2-3 cm. in diameter in containers of various 

 sizes. The glass tubes were closed over the bottom with musHn, and 

 each contained a single snail. Each day these tubes were hfted from 

 the water and replaced two or three times in order to secure complete 

 mixing of water. Even so, the contained snails grew approximately 

 the same regardless of the volume of water with which they were in 

 contact through the muslin screen. In one instance the growth was 

 the same in such a muslin-bottomed tube as compared with that of a 

 snail in a corked tube which prevented all exchange between the 

 inner and the surrounding water. From these experiences he con- 

 cluded that Semper's explanation would not hold and that the size 



