204 A. G. Wetherby—Distribution of Fresh-water Mollusks, 
influence of Archean lands upon the subsequent moulding and 
forming of the continent, whose final systems of drainage, and 
all the stages of development leading to them, were determined 
by this early and stable region, which had its representative 
areas on both sides of the incipient uplift, and at comparatively 
isolated points over the great central basin; areas aroun 
which clustered, throughout the history of continental progress, 
the geological activities that determined eve ry thing. 
t seems desirable, in discussing the variations above hinted 
at, to remember that there must have been a far greater impe- 
creatures the vicissitudes accompanying a ce into bod- 
ies of flowing water. Such changes of s nally of 
habitat, were among the last possibilities of oe growth, 
because it was only in connection with the later grand move- 
ments associated with terrestrial evolution, that present sys- 
tems of drainage became possibilities. It is likewise true, that 
h 
range of forninons. ravers will be sreatek Threagh the more 
extensive erosion. Third, because in mountainous regions 
there is an increase of probability that saneval deposits will 
fall in the path of streams, which will effect changes in the 
water, causing enone stunting, or extraordinary devel 
ment, of given for Fourth, because the influx of side 
streams, bearing the Water of mineral springs, will saa to these 
effects. Fifth, because here we have the maximum of extremes 
in rate of current, and consequently the maximum of capacity 
to transport sediments that may act favorably or unfavorably 
upon the various creatures inhabiting these streams. th, 
(ax 
because of the probability that these mollusks have been prop: 
agated down stream, to the limit of favorable conditions—@ 
eS Ses Seen Me es Lo easy. 
