ADJUSTMENT OF ORGANISMS TO ENVIRONMENT 369 

 I. ADJUSTMENT IN PLACE AND TIME. 



We go to the woods for squirrels, to the marsh for snipe 

 and to the lake for fish. We do not expect to find either in 

 the place of the other; indeed, we know they could not live 

 if they exchanged places. If we likewise go to the beach 

 for sand or to the mine for gold, we know that either might 

 exist as well unchanged if put in the place of the other. The 

 gold or the sand may have lain unchanged for ages, but 

 squirrel and snipe and fish have developed with their 

 environment, and are developing still. 



It is not everywhere in the woods that we find squirrels. 

 They have their own particular haunts. They like the nut- 

 bearing trees, and shun the thorny locusts. They like cer- 

 tain bird neighbors and dislike others. In the water we 

 find pickerel and top-minnows feeding at the surface, cat- 

 fishes and mud-minnows feeding on the bottom, and other 

 fishes foraging between; different forms of life at different 

 levels; and likewise, passing out from deep water shoreward 

 we find that every change of forage and shelter brings with 

 it its own peculiar forms of life. The more closely we look 

 into any environment the more we see of small and seques- 

 tered species, restricted in range and peculiar in mode of 

 life, segregated into definite and sharply delimited haunts. 

 The physical conditions of life in the water are still simple, 

 but with the multiplication of individuals and differentia- 

 tion of species, by reason of the stress of competition on 

 every hand, the biological conditions have become severe. 

 Only a few of the stronger and larger species frequent the 

 open water, and these only when they have attained 

 maturity; the great majority of the lake's inhabitants dwell 

 in some restricted sphere. The great sturgeon may roam 

 the lake at will, but the little darters, and infant sturgeons 

 as well, must keep to shelter. 



A new study, appropriate at this point in the course, has been added 

 on page 525. 



