GENERAL FACTORS CONDITIONING AGGREGATIONS 69 



that stones thrown at them could not stop them nor alter the posi- 

 tion of a single animal. They would make the proper movements and 

 the stone would roll off; all the snakes in this lump were common 

 garter snakes {Eutaemia sirtalis L.). 



"The second time I noticed a ball of black snakes rolling slowly 

 down a steep hillside on the bank of the same river. Some of the 

 snakes were of considerable length and thickness and as I noticed 

 clearly, kept together by procreative impulses." 



Lunar periodicities. -Such breeding aggregations are much more 

 important in fresh-water and land forms, with whom the surround- 

 ings are more injurious to shed sperm or eggs, but they do occur 

 among marine animals. With marine organisms the most spectacular 

 expression of breeding aggregations is to be found in the case of the 

 large number of animals whose breeding rhythms coincide to some 

 extent with lunar periodicities. The Hterature on this subject is ex- 

 tensive (Woodworth, 1907; Fox, 1923; Legendre, 1925; B. H. Grave, 

 1922, 1927); but while the facts are plain enough, the fundamental 

 causal relations remain unknown. One illustration must suffice, 

 based on the account given by Lillie and Just (191 3) for the swarm- 

 ing of the sea worm Nereis limbata in waters around Woods Hole. 



Nereis limbata has its swarming period only after twilight. Each 

 run begins near the time of the full moon, increases to a maximum 

 during successive nights, and sinks to a low point about the time of 

 the third quarter, again rising and falling to extinction shortly after 

 the new moon. They appear in four periods or cycles during the 

 summer, corresponding to the lunar cycles in the months of June, 

 July, August, and September. 



Only fully mature animals swarm. The swarming begins shortly 

 after twilight and lasts for only an hour or so. The swarming animals 

 are attracted by the light of a lantern. Males appear first, darting 

 through the water in curved paths in and out of the circle of the 

 hght. Females are fewer in number and swim more slowly. The 

 males outnumber the females hundreds to dozens. In the next few 

 minutes the numbers increase, waning again after about three- 

 quarters of an hour. 



New females appear each night, but some males may presumably 



