ORGANIZATION AND GROWTH 229 



made only by Semper. 1 Semper started with the idea ex- 

 pressed by H. Spencer in his Principles of Biology: "All 

 who have had experience in fishing in the Highland lochs 

 know that where the trout are numerous they are small, and 

 where they are comparatively large they are comparatively 

 few." Semper studied the effect of the quantity of water 

 upon the longitudinal growth of Lymnaeus stagnalis. Elodea 

 canadensis furnished in abundance served as food. The 

 stiff shells of the Lymnaei allowed an accurate measurement 

 of the amount of growth. Semper found that when the 

 food was equally overabundant the Lymnaei grew the more 

 rapidly, within certain limits, the greater the amount of water 

 in which they were contained. With an increase in the volume 

 of water from 100 c.c. to 500 c.c. the longitudinal growth 

 increased rapidly. As soon as the volume of water amounted 

 to 5,000 c.c., a further increase had no effect upon growth. 

 After sixty-five days in one experiment, a shell 



in 100 c.c. of water was 6 mm. long 



in 250 " " " " 9 " " 

 in 600 " " " " 12 " " 

 in 2,000 " " " " 18 " " 



Semper is inclined to believe that the dependence of the 

 growth upon the amount of water present is attributable to 

 the presence of a substance in the water "without which the 

 other conditions for growth, even when present, can have no 

 effect upon growth." In his book on The Conditions of the 

 Existence of Animals, he describes further experiments in 

 this direction: 



The salts in the water, the presence of which can be demon- 

 strated chemically, cannot be the cause of the stunted development. 

 With the friendly aid of a chemist, Professor Hilger of Erlangen, 

 I repeated my experiments with distilled water, or with water in 



i C. SEMPEE, " Ueber die Wachstumsbedingungen des Lymnaeus stagnalis," 

 Arbeiten aus dem Zool. zoot&m. Institut in Wilrzburg, Vol. I (Wurzburg,1874), p. 137; 

 and Die naturalischen Existenzbedingungen der Thiere, Part I (Leipzig, 1880), p. 195. 



