PERIODICITY OF ABUNDANCE AND SCARCITY. 
391 
defined, distinguished by the association of certain mollusks, with a 
preponderance of particular trees and other vegetation. The weakest 
terrestrial forms, are, however, always restricted to the more extreme 
localities, while the more advanced species occupy the more desirable 
areas of lesser altitude ; the stronger aquatic species similarly inhabit 
the shallower waters, driving the weaker species or individuals into 
those of greater depth. 
Periodicity, or cycles of abnormal abundance or rarity in the life of 
a species or group, also influences the increase and diffusion of all 
organisms. A group, species or variety may more or less rapidly attain 
a maximum of abundance and therefore dispersive power from no 
apparent assignable cause, though probably owing to circumstances 
being exceptionally favourable to the mode of life or constitution of 
the particular species affected ; this period of abnormal abundance and 
consequent enforced dispersion may continue for a variable period, 
but sooner or later a decline sets in and the species, if not a dominant 
one, is gradually reduced in numbers until it may ultimatel}' become 
quite rare or even extinct. 
This phenomenon in a local and restricted wa}" is known to most 
field-naturalists ; a pond near Binningham formerly swarmed with 
Velletia lacustris var. compressd, but this abnormal abundance after a 
time began to wane, until the species became quite rare or had arrived 
at its minimum in that place. Amphlpeplea rjlntinom, has been 
especially noticed to be affected by this periodicity, and it may well be 
that Dreissensid polpmorph i owes its rapid diffusion in Europe to 
the advent of a great maximum period of abundance, paralleled in 
an evanescent wvay by the irruptions of the immense flocks of Pallas’s 
sand grouse into Europe, or the migrations of the hordes of Lem- 
mings, and if so we .shall doubtless observe in due course in 
our Dreisspnsia a diminution of its excessive numbers as its period 
of prosperity wanes, as the invasions or irruptions of a weaker species 
within a region occupied by a stronger race, filling approximately the 
same sphere of life, can only be of a temporary character, as the 
stronger species will eventually reassert its superiority and dispossess 
the feebler form, which will more or less (quickly di.sappear. Possibly 
the fossil remains of Dreissensid in Europe may point to a former 
period of prosperity and extended diffusion, or may be merely indica- 
tions of its continuous existence in the European region within which 
it probably originally emanated. 
