THE ORGANISM AND ITS ENVIRONMENT 381 



singly or in groups, or the size of the individuals may be smaller 

 than usual for that particular species. In a closed system, such as a 

 land-locked pond, the migration of fish to new feeding grounds is 

 impossible and the individuals may be quite small, due to excess 

 number, or over-population. Game wardens recognize this relation 

 between numbers and size and often remove the excess fish and 

 transfer them to ponds or lakes which have been under-populated, 

 so far as that species is concerned. 



Even if undisturbed by Man, no animal community remains 

 unchanged for any considerable period of time. Climatic changes 

 occur, for instance, dry or wet seasons, that may be only of brief 

 duration, but in effect disturb the plant life upon which the animals 

 depend, or reduce or increase numbers of various animal species 

 that make up the community. Other and less sharply defined influ- 

 ences are constantly affecting the life of the community, invasions 

 of different species from without, erosions or other changes in the 

 soil, accumulation of soil by decay of plant and animal life, and so 

 on. Thus animal communities change, species disappearing or mi- 

 grating, to be replaced by other types or to reappear again. A given 

 locality may therefore at one time contain a preponderance of one 

 type of animal and at another time contain a preponderance of 

 another type. An example on the broadest possible scale is afforded 

 by the known fact that in an earlier geological age enormous rep- 

 tiles that have long been extinct occupied a prominent place among 

 animals. On a small scale one may observe a succession of changes 

 in an animal community within an aquarium. If dry hay is olaced 

 in water and seeded with a bit of mud taken from a pool, in a few 

 days bacteria are present in large numbers, obtaining their sub- 

 sistence from the decaying plant material. Presently small ciliate 

 Protozoa appear and increase in numbers, living upon bacteria. 

 Larger varieties of ciliates become prominent, feeding on smaller 

 ciliates. Eventually, as the essential food decreases the community 

 dies out. If an aquarium is set up containing the proper proportion 



