474 EFFECT OF COMPLEX AGENTS ON GROWTH [CH. XIX 



Now it is a common observation of naturalists that there is 

 frequently a relation between the size attained by a developing 

 organism and the extent of the medium in which it is develop- 

 ing. Animals reared in aquaria rarely attain the size of their 

 fellows developing in out-of-door ponds. Even in nature, fishes 

 of a given species living in small or densely inhabited ponds 

 are smaller than fishes of the same species inhabiting large or 

 sparsely populated ponds. According to SEMPER ('74) the 

 experiment was tried of transplanting the very small chars 

 from a small lake in the Maltathal, Carinthia, Austria, where 

 they were very abundant, to another lake where there were few 

 fish. The transplanted chars soon became three times the size 

 of the parent stock. 



In other experiments it has often been observed that tadpoles 

 reared in the laboratory, although well fed, never attain the 

 size of those living free. SIEBOLD * found that Apus reared in 

 a small vessel grew no longer than 7 or 8 mm. instead of 50 

 or more. The pond-snail, Limnsea stagnalis, has been made 

 the special object of experimental dwarfing. HOGG ('54) 

 found that when confined to a small, narrow cell they ac- 

 quire, even after six months, only the size of normal animals 

 two or three weeks old. HOGG'S conclusions no doubt ap- 

 peared crude to the physiologist of the last decade. He said 

 that the snail grows "to such a size only as will enable it 

 to move about freely, thus adapting itself to the necessities of 

 its existence." 



SEMPER ('74) next undertook a more thorough set of experi- 

 ments upon these animals. He kept the snails in vessels con- 

 taining different quantities of water to each individual, and 

 found that as the quantity of water is diminished from 4000 cc. 

 or 2000 cc. to 100 cc., a diminution in the rate of growth occurs. 

 Increasing the number of individuals in a vessel has the same 

 effect as diminishing the volume of water. The relation of 

 volume of water per individual to length of shell at the end of 

 eight or nine weeks is given in Fig. 134. This curve shows 

 that as the volume increases from 100 cc. to 800 cc., the average 

 length of the shell of the snail doubles. The result SEMPER 



* Cited from SEMPER ('74), 



