INTRODUCTION 3 



units of environment. These more nearly stable units, the lakes 

 and ponds, are often rich in life. They are exceptionally favorable 

 for study and have been extensively investigated both in Europe 

 and in this country. 



The temporary water body, a puddle or pool, whatever its area, 

 affords only conditions for transient existence that are sometimes 

 irregular in their recurrence and sometimes present themselves 

 with considerable regularity. They are fitted for organisms that 

 reproduce very rapidly during the favorable season and also have 

 special means of tiding the species over the unfavorable period. 

 Purely temporary water bodies, such as pools that form in hol- 

 lows after a heavy rain or in a wet season, develop little if any 

 life. Such places on poor soil are most barren of all; the aquatic 

 life increases with the fertility of the soil, the age of the water body, 

 and the consequent accumulation of organic debris. Residual 

 ponds, water bodies in which the drying out is more gradual and 

 often incomplete and in which a central area may be protected from 

 complete desiccation by vegetation or proximity to the general water 

 level, afford conditions at the opposite extreme. The wide stretches 

 of lowland subject to periodic overflow from great inland rivers like 

 the Illinois, Missouri, and Danube in certain regions, develop a rich 

 flora and fauna which equals or exceeds- in abundance that found 

 under other circumstances (Antipa, Forbes). Similarly among 

 ponds adjacent to a lake basin the permanent are poorer than 

 those which dry out for a time (Shelf ord). 



The smaller water body presents nearly uniform conditions 

 throughout and therewith also a single series of inhabiting organ- 

 isms. The entire area falls within the shore or shallow water 

 zone which is limited to such parts as support fixed plants. In 

 this general region are readily distinguished two zones, (a) that 

 of the emergent vegetation where the larger plants reach conspicu- 

 ously above the water level and constitute the dominant feature 

 to the eye, and (2) that of submerged vegetation in which the 

 plants rarely project at all above the surface and in consequence 

 the water itself dominates the view. Both of these regions may be 

 subdivided on the basis of the particular form of vegetation which 

 is common in a given portion. In a swamp these regions are often 



