84 THE LIFE OF THE MOLLUSCA 



Nevertheless, when such millions of fry are pro- 

 duced by even a single parent (see infra, p. 89), and 

 all committed to a sole agent for dispersal, it is 

 evident that the greater number will be carried in 

 one direction and to a single area, and that the 

 shorter the time before they settle down the greater 

 will be the number thus congregated together. To 

 this fact must be attributed the large assemblages of 

 one species, such as the Common Shore Mussel, and 

 not to any wish for a gregarious or social life after 

 the manner frequently displayed by animals higher 

 in the scale of organization. To their limited 

 capacity for locomotion is similarly attributable 

 the colonies of Land Mollusca. Each lives for itself, 

 competing with its fellows for food, the abundance 

 or dearth of which principally determines their 

 numbers, without thought of co-operation or mutual 

 assistance. 



What the currents will effect for marine Mollusca, 

 the running water of streams and rivers, especially 

 in times of flood, will do for the freshwater ones and 

 more, since during floods not merely the eggs and 

 offspring, but the animals themselves, will be borne 

 away to fresh spots by the rushing current, and, as 

 the waters subside, be left in ponds and ditches that 

 are otherwise disconnected. The colonization of 

 permanently isolated pieces of water, and even the 

 dew-ponds on high hills, apart from accidental im- 

 portation by man, is effected by other agencies of 



