ORGANIZATION AND GROWTH 229 
made only by Semper.’ 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 cc. to 500 cc. 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 cc. of water was 6 mm. long 
in 250 ics + “ ee 9 “ce ee 
in 600 “ 6“ “ce “ 12 “ “ 
in 2,000 (73 co <3 (73 18 (73 73 
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 
1C, SpmpPER, “ Ueber die Wachstumsbedingungen des Lymnaeus stagnalis,” 
Arbeiten aus dem Zool. zootom. Institut in Wirzburg, Vol. I (Wirzburg, 1874), p. 137; 
and Die naturalischen Existenzbedingungen der Threre, Part I (Leipzig, 1880), p. 195. 
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