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 ahedent 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 
