CHAPTER IX 

 STIMULATION OF GROWTH BY CROWDING 



Having sketched in some detail the harmful effects resulting from 

 the crowding of many animals into a relatively small space, it is now 

 possible, with a better perspective, to look into the more recently ac- 

 cumulated evidence that harmful results do not necessarily follow 

 the formation of such aggregations and that they are often useful and 

 even necessary to the welfare of the individual. 



The extent to which the phenomenon of aggregation affects the 

 rate of growth in a positive manner has been relatively little in- 

 vestigated. In the work of Colton (1908) upon Lyninaea it will be re- 

 called that he found crowding generally decreased the rate of growth 

 in snails. He found, however, that the snail faeces, if washed free of 

 easily soluble material and placed in weak solutions with snails, 

 tended to increase the rate of growth. With concentrated solutions 

 of faeces the results were reversed. Similarly, weak solutions of urea 

 favored snail growth, though stronger solutions retarded it. 



Popovici-Baznosanu (192 1) also found that under certain condi- 

 tions snails grew more rapidly in stagnant water, conditioned by the 

 snails, than in fresh water. In short experiments (1914) 10 young 

 Lymnaea attained a length of 9.5 mm. in fresh water while those 

 living in stagnant water grew to 10 mm. Later he tested this effect 

 for a longer period. The young Lymnaea from three egg masses were 

 placed in three culture dishes of identical dimensions as regards vol- 

 ume and surface of water; after a long sojourn, when the water was 

 thoroughly snail-conditioned, Popovici-Baznosanu took half of the 

 individuals and placed them in better conditions of existence, in 

 culture jars with a large volume and a relatively large surface, and 

 containing fresh pure water. Elodea was used as food both for those 

 in the stale and those in the fresh water, and in the same quantity 

 for both. After 106 days the results were as given in Table V. 



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