202 THE SCIENTIFIC RESULTS OF V 



maintained, so that, with local exceptions here and 

 there, those that survive to maturity in the struggle 

 for existence merely replace those which have gone 

 before. Many thousands of young perish, serving as 

 nourishment for other lower and higher organisms, 

 whilst the total number of mature organisms remains 

 the same. 



Take as an example the microcosm constituted by 

 a pond in which carp are cultivated, as in Germany, 

 where these fish are valued as food.^ Such a pond is 

 allowed to remain dry every fourth year, when it is 

 cleansed and puddled. Water is then allowed to run 

 in, and a given number of one-year-old carp are placed 

 in it. After three years these are taken out and sold 

 as food. In one such pond 30,000 young carp have 

 been observed constantly to yield at the end of three 

 years 20,000 kilogrammes of marketable carp ; when 

 more than 30,000 young carp have been placed in the 

 pond, no greater yield has resulted, 20,000 kilos, 

 weight of carp-flesh is the result of that pond's activity 

 after three years. The food of the carp consists of 

 delicate vegetable growths, of insects, and other minute 

 aquatic animals. These hatch from eggs and spores 

 introduced with the water into the pond, and the pond 

 will only carry such an amount of this food as will in 

 three seasons produce 20,000 kilos, of carp-flesh. It 

 is further found necessary to keep with these carp in 

 the same pond a few pike, which prey upon the carp 

 to a certain extent. The carp-culturists know how 

 many pike to introduce. A few act beneficially in 



1 I borrow this illustration from Professor Mobius of Kiel, 



