302 THE EFFECT OF FOOD 



bodies, which differ in different organisms. If they 

 come into contact again with the tissues from which 

 they have been expelled, they retard the growth of these 

 tissues, but if with other tissues with which they have 

 no direct chemical relation or association, they may 

 under certain circumstances stimulate them to in- 

 creased growth. 



The effects of products of metabolism upon growth 

 have been tested at considerable length in the case of 

 certain Molluscs. At least it is to this influence that the 

 results obtained by Karl Semper and by De Varigny in 

 their experiments on Limncea stagnalis, the common 

 pond snail, ought, in my opinion, to be ascribed. 

 Semper * found that if various numbers of the small 

 snails were placed in equal volumes of water immedi- 

 ately after hatching, and were kept there under other- 

 wise equal conditions as to food, temperature, etc., for 

 about two months, then the size to which they attained 

 was by no means equal, but varied in more or less in- 

 verse proportion to the number of snails present. In 

 four very consistent experiments, the numbers of snails 

 placed in volumes of 2000 cc. of water were in each 

 case respectively 1, 5, 10, and 20, or each snail obtained 

 respectively 2000, 400, 200, and 100 cc. of water. The 

 lengths attained by the snails after two months' growth 

 are given in the table below. 



Here we see that snails allowed to grow singly in the 

 2000 cc. vessels of water attained to more than three 

 times the size of those grown in twenties. This was not 

 merely a question of nutrition, as the amount of food 



* Arb. a. d. Zool. Inst. in Wurzburg, i. p. 137, 1874; also " Animal 

 Life," ed. 4, p. 51. 



