358 ANIMAL AGGREGATIONS 



to the congregation of the two sexes during the breeding season. 

 Less than one-third of these forms possess sufficient social appetite 

 to allow them to be classed as social animals in the usual sense of the 

 term. Definite racial survival values have been demonstrated for 

 about one-half, again excepting ordinary bisexual breeding relations; 

 and of this half, about two-thirds are usually called "non-social." 

 Even this brief survey shows that in addition to being a widespread 

 phenomenon, taxonomically considered, survival values frequently 

 accrue from animal aggregations in a state of nature, and often much 

 below the level of group integration usually called "social." 



The field naturalist, interested in observing a wide range of ani- 

 mal life, is familiar with the widespread occurrence of aggregations. 

 Inland waters are notoriously poorer in population than is the sea; 

 but in California, during the breeding season, I have seen ponds 

 paved with the pebble-like clusters of salamander eggs. In mid- 

 Great Salt Lake our boat ploughed through surface-covering masses 

 of aggregated Ephydra flies that rose in choking numbers. Aldrich 

 (191 2) calculated 370,000,000 of these were to be found along every 

 mile of Salt Lake beach. In the nearby mountain ponds of Utah 

 aggregations of ostracods of the size of a walnut were to be found, 

 at times occupying a portion of each cow track with which the bot- 

 tom of the ponds were stippled; and similar collections of annelid 

 worms occur in Indiana ponds. The collections of Hydra in favor- 

 able spots along Lake Michigan remind one of the abundance of 

 marine organisms; and in some portions of spring-fed watercress 

 swamps the supply of Planaria dorotocephala seems exhaustless. 



Along the seashore, in such favorable locations as part of the 

 Cahfornia coast, the supply of animal life is appalling. One cannot 

 step on the rocks exposed at low tide without crushing sea urchins, 

 sea anemones, barnacles, or mollusks. Even in the less prolific re- 

 gions around Cape Cod every available rock or solid timber washed 

 by the tidal currents is the base for a densely packed aggregation, 

 composed of many or of few species. Favorable bottom areas are 

 similarly packed; and Mytilus and Crepidula fornicata, if proper 

 substratum be wanting, form chains of animals, attaching to each 

 other in the absence of solid objects. A suitable bit of mud flat may 



