HARMFUL EFFECTS OF CROWDING UPON GROWTH 117 



of developing eggs is found to produce about the same degree of re- 

 tardation as is furnished by crowding. Under these conditions the 

 dwarfing effect must be a result of toxic materials accumulated in 

 the water. The general importance of these results is enhanced be- 

 cause of the fact that they have been obtained from animals grown in 

 running rather than in stagnant water. There was an indication of a 

 condition of optimum crowding^ in the early experiments which was 

 not sustained by later work, although specific experiments designed 

 to test this point were not attempted. 



Peebles (1929) has taken up the problem of effect of numbers 

 present upon the rate of cleavage of echinoderm eggs, and upon the 

 rate of growth of arms of plutei, in the light of developments in 

 tissue-culture work. She finds, as did Vernon (1895) and Springer 

 (1922), that embryo-water contains substances which check growth, 

 but adds the observation that some of the inhibiting effect is counter- 

 acted when living larvae are present. She produces evidence that the 

 growth-inhibiting substances are associated with the lipoids and 

 that, after their removal, growth-promoting substances can be dem- 

 onstrated to be present. These latter will be discussed in chapter ix. 



The relation between the size of the effective environment and 

 that attained by the animals living therein has more than laboratory 

 interest. The belief is widespread that fish grow larger in large lakes 

 than in small ones. Pearse and Achtenberg (1920) report such a 

 correlation between size of lake and size of contained yellow perch. 

 This correlation is not uniform, for numerous exceptions could be 

 cited; for example, Jewell and Brown (1929) find no such relation- 

 ship holding between size of fish and the size of the small Michigan 

 lakes in which the fish live. 



Hesse (1924) states that the same relation holds for mammals with 

 regard to the size of available range: those living on small islands 

 attain a smaller adult size than related forms on larger bodies of 

 land. In many cases the reduced amount of available food in the 

 smaller habitats has been recognized as being sufficient to explain 

 the observed phenomena. Semper (1879) critically discussed this 



• Kawajiri reports that the survival-rate increases as the number of fry in a box 

 increases. 



