550 



THE CAUSES OF EVOLUTION 



which Uved a thousand years, if a single seed were produced once 

 in a thousand years, supposing that the seed were produced and 

 never destroyed and could be ensured to grow and germinate. 

 Lighten any check, mitigate the destruction ever so little, and 

 the number of the species will almost instantaneously increase 

 to any amount." 



Black bass adults 

 jBlack bass young 



Fig. 305. — Representation of the equilibrium in the food relations of a 

 community of organisms in a pond. 



Algse are simple green plants; mussels, Sphaeridae, Physae, and Limmoe are mollusks; 

 and amphipods, entomostraca and crayfishes are crustaceans. The arrows point from the 

 food organisms to the feeding organisms. 



Modern studies in ecology have further emphasized the intricacy of the inter-relation- 

 ships between organisms that was discussed by Darwin in connection with his formulation 

 of natural selection. Professor Shelford writes as follows in explanation of this diagram: 



"Any marked fluctuation of conditions is sufficient to disturb the balance of an animal 

 community. Let us assume that because of some unfavorable conditions in a pond during 

 their breeding period the black bass decreased markedly. The pickerel, which devours 

 young bass, must feed more exclusively upon insects. The decreased number of black 

 bass would relieve the drain upon the crayfishes, which are eaten by bass; crayfishes would 

 accordingly increase and prey more heavily upon the aquatic insects. This combined 

 attack of pickerel and crayfishes would cause insects to decrease and the number of pickerel 

 would fall away because of the decreased food supply. Meanwhile the bullheads, which 

 are general feeders and which devour aquatic insects, might feed more extensively upon 

 mollusks because of the decrease of the former, but would probably decrease also becauss 

 of the falling-off of their main article of diet. We may thus reasonably assume that the 

 black bass would recover its numbers because of the decrease of pickerel and bullheads, 

 the enemies of its young. A further study of the diagram shows that a balance between 

 the numbers of the various groups of the community would soon result. Under other 

 circumstances, such as the extinction of the black bass, the resulting condition would be 

 entirely different from the original one, but a balance between supply and demand would 

 nevertheless finally be established." 



(From Shelford, "Animal Communities in Temperate America," copyright, 1913, 

 by Geographic Society of Chicago, reprinted by permission.) 



