1908.] NATURAL SCIENCES OF PHILADELPHIA. 411 



that are most easily observed are those of relative rate of growth and 

 of relative fertility. Although naturalists for many years have 

 recorded cases of this sort, few have undertaken an experimental study 

 of the factors concerned. 



External conditions modifying one structure of an organism have 

 usually been found to be correlated with similar modifications in other 

 organs, yet few correlations between dissimilar physiological processes 

 have ever been observed, although most naturalists hold them to be 

 present. 



In studying the effect of confinement on organisms, nearly all 

 investigators have chosen the Pond Snail as an animal admirably 

 adapted to their purpose. Perhaps it is largely due to the abundance, 

 to the hardiness, to the rapidity of growth and above all to the extreme 

 sensibihty of Pond Snails to any slight change in environment that 

 they have been so universally chosen. 



II. Historical. 



Jebez Hogg ('54) discovered that Lymnoea confined in small aquaria 

 were much smaller than their brothers of the same egg case raised in a 

 large one. The latter were full grown and had produced young which 

 were as large as the former at the end of six months. Hogg attempted 

 to explain this phenomenon by saying that the snail had the power of 

 "adapting itself to the necessities of its existence." 



Carl Semper ('79) did not consider this as an explanation. He be- 

 lieved that there was a definite factor that would cause dwarfing. This 

 led him to perform a series of experiments with various sized containers. 

 The conclusion that he arrived at was as follows: that there was a 

 chemical in the water (he had the water analyzed, but nothing was 

 discovered) that stimulates growth without actually contributing to 

 it, but yet is essential, "like oil to a steam engine." 



The next investigator to enter this field was E. Yung ('78, '85). He 

 proceeded to raise tadpoles from the egg in various sized and shaped 

 containers. He found that those with the greatest area exposed to the 

 air held the largest tadpoles after a certain length of time. The 

 obvious conclusion w^as that the dwarfing was caused by lack of 

 aeration. 



Stimulated by the experiments of Semper and Yung, De Varigny ('94) 

 made an attempt to solve the problem by returning to Lymncea. After 

 a hundred or more very careful experiments, he did not dare venture 

 any very definite conclusion, but thought that the dwarfing of these 

 Pond Snails was caused by lack of exercise. 



