ANIMAL AGGREGATIONS 



393 



gest that, while integration in the organ- 

 ism is biological, integration in the popula- 

 tion is statistical, and that therefore the 

 two are not in any sense comparable. This 

 would be fallacious. The operations of 

 populations are equally biological, but typi- 

 cally need to be expressed in statistical 

 terms. Evolution is recognized as one of 

 the truly great concepts of biology, yet the 

 theory of evolution is populational in 

 character and is best treated statistically. 

 The population has attributes of its constit- 

 uent organisms— reproduction, death, me- 

 tabolism, irritability, growth and diflFeren- 

 tiation, a genetic make-up, environmental 

 adaptation and adjustment— but in its exist- 

 ence as an integrated group these attri- 

 butes appear as the aggregate (or some 

 modification of this) of individual re- 

 sponses. From this there emerges an inte- 

 grated unit that attains a new biological 

 status, which can be studied and analyzed. 

 This new level has properties uniquely its 

 own in addition to those of its parts. 



Many of these properties take statistical 

 form (see Chap. 18); for example, a birth 

 rate or a death rate, or a gene-frequency 

 heredity, are population attributes depend- 

 ing on the fact that organisms reproduce, 

 die, and have a genetic constitution; but 

 meaningless when applied to an organism. 

 If the population attains a certain level, 

 it may become by definition "social," even 

 emerging with a division of labor among 

 its members (p. 435). This reaches its 

 height of development among the social 

 insects, in which castes are actual mor- 

 phological expressions of populational divi- 

 sion of labor with a whole series of phys- 

 iological, anatomical, and behavioristic 

 adjustments superimposed upon the gen- 

 eral population features that in turn are 

 superimposed upon organismic features 

 (for a discussion of social populations, see 

 Emerson, 1939a). This question receives 

 special attention in the two chapters that 

 follow. 



23. ANIMAL AGGREGATIONS 



Contagious distributions are the rule in 

 nature (cf. p. 365). Sessile as well as mo- 

 tile animals settle or collect in favorable lo- 

 calities, especially when the optimal niches 

 are limited in extent. Often the aggregations 

 do not occupy all the space that appears 

 equally favorable. The observations of 

 naturalists demonstrated long ago that the 

 collecting together of animals is a common 

 occurrence, and aggregations often re- 

 ported include the following among many 

 others: 



1. Hibernating or aestivating groups 



2. Overnight aggregations 



3. Collections about food 



4. Aggregations on suitable but limited 

 substrate 



5. Concentrations in shelter niches 



6. Breeding aggregations 



7. Partial or complete family groups 



A fairly complete list of animal aggre- 

 gations is given by Allee (1931, Chap. II). 

 The more dramatic assemblages have at- 

 tracted especial notice, and naturalists' 

 records abound in both casual mention and 

 more careful description of unusually large 

 or dense animal aggregations at all phylo- 

 genetic levels. 



Oligotrophic lakes and rivers are noto- 

 riously less dense in populations than are 

 bays and sounds along marine coasts, but 

 such paucity does not extend to all inland 

 waters. Near San Francisco, during the 

 breeding season, ponds may be paved 

 with pebble-like clusters of salamander 

 eggs. In mid-Great Salt Lake, where the 

 water is over 22 per cent salt, one finds 

 surface-covering masses of aggregated Eph- 

 ijdra, and, in season, some 370,000,000 of 

 these flies are to be found along each mile 

 of Salt Lake beach. In nearby mountain 

 ponds of low salt content, ostracods of the 

 size of pin heads aggregate in groups as 

 large as a walnut, one aggregation to each 

 of the cow tracks that stipple the pond 

 bottoms. Similar collections of annelid 

 worms may occur in Indiana ponds. The 

 concentrations of hydra, in favorable places 

 and seasons, along Lake Michigan almost 

 resemble the abundance of small marine 

 hydroids in similar habitats along the 

 shores of Cape Cod; and in some portions 

 of spring-fed water cress swamps, the sup- 

 ply of planarian worms seems inexhausti- 

 ble. 



Some of us have gathered adult may 



