DAPHXIA POND. 231 



Environment may mean any of the following conditions: Physical 

 conditions, temperature and moisture, so as to make food abundant and 

 the conditions favorable to life. In such a case the species would mul- 

 tiply rapidly, with little tendency to variation from the normal condition. 

 These conditions may be such as to make food scarce, make life a struggle, 

 and kill off the great majority of the organisms of a species. In such a 

 case there is marked tendency to variation. Those characters or traits 

 most useful or helpful in the struggle will be selected, and organisms dif- 

 fering from their ancestors in some ways will be the result. 



Again: in addition to the above two cases, and modifying either, 

 there may be the presence or absence of natural enemies, which prey 

 upon the organisms, increasing or reducing in numbers accordingly. 

 Where food is most abundant and enemies practically absent there is 

 great multiplication of numbers. Illustrations of these conditions are 

 to be seen in America in the English sparrow, the San Jose scale, the 

 codling and gypsy moths, and other noxious insects. Where food is 

 scarce and enemies abundant there is either great variation or extinc- 

 tion of species, or both. Under such circumstances the struggle is 

 keenest and most severe, those least able to survive are killed, and the 

 resulting and living specimens are likely to be strong and hardy, unlike 

 their ancestors, continuing to vary in structure so long as the hard con- 

 ditions exist. 



Let us make application of these principles to the life as we find it in 

 Daphnia Pond. 



Vegetable Life. Trees are absent. As no trees in the region other 

 than willow shrubs can live in water the pond must be older than any 

 trees growing near it. 



When water collects in any place it is immediately invaded by cer- 

 tain forms of vegetaole life, water plants. In sustaining life these plants 

 begin to fill the pond. Their roots sink into the soil to hold the plant. 

 Their stems become so thick and matted that whatever silt is brought into 

 the water is held, and is not permitted to run out. The pond is gradu- 

 ally filled in, the plants in living, slowly make living impossible, and the 

 result is the extinction of the pond and the death of both its animal and 

 vegetable life. 



Daphnia pond admirably illustrates the method by which a pond is 

 filled. In the center is a small space of open water, twenty feet deep. 

 This is bordered by a fringe of yellow water lilies, whose roots are deep 

 in the mud, and whose leaves reach up through five or six feet of water 

 to the surface. Among these are matted masses of lower forms of 

 vegetable life. Outside, in shallower water, the rushes and cattails hold 

 sway, their decaying leaves and stems each year adding to the decayed 

 vegetable material. Nearer the shore the sedges have taken hold, and 

 formed large hummocks, sufficient to bear the weight of a man. Along 

 shore willows have taken firm hold on the soil. 



This tangled mass of hydrophytic vegetation affords abundant hiding 

 place for various forms of animal life, and at the same time supplies 

 food for them, as testified by their great numbers. 



The glaciated ridges adjacent have in very recent years been cleared 



