Geographical Distribution of Certain Fresh-Water Mollusks, 157 
In this consideration due weight must be given to the great influence 
of Archean lands upon the subsequent moulding and forming of the 
continent, whose final systems of drainage, and all the stages of de- 
velopment leading to them, were determined by this early and stable 
region, which had its representative areas on both sides of the incipi- 
-ent uplift, and at comparatively isolated points over the great cen- 
tral basin; areas around which clustered, throughout the history of 
continental progress, the geological activities that determined every- 
thing. ye 
It seems desirable. in discussing the variations above hinted at, to 
remember that there must have been a far greater impetus given them, 
when changes in drainage brought to these creatures the vicissitudes 
accompanying distribution into bodies of flowing water. Such 
changes of station, and finally of habitat, were among the last possi- 
bilities of continental growth, because it was only in connection with 
the later grand movements associated with terrestrial evolution, that 
present systems of drainage become possibilities, It is likewise true, 
that at no time since any drainage became possible to the continent, 
in streams large enough to contain a shell fauna, has there been such 
a complication of circumstances favorable to the local variation of that 
fauna, and the consequent establishment of varieties as now. For while 
it is a well determined fact in geology, that with the progress of conti- 
nental evolution, the complexity of the characters of strata increased, 
it is also true that each of these new features would become a factor 
of importance in modifying the character of streams flowing through 
the land, and would, for this reason, aid in changing the nature of the 
mollusks inhabiting them; and these facts reach their greatest 
development in mountain regions, for the following among other causes 
that may be enumerated. 
First, it is in mountainous regions that streams cut their way 
through strata of the most heterogeneous character, partly owing to 
the effects of metamorphism and other disturbing causes upon strata 
that may have been, originally, more homogeneous. Second, because 
even where metamorphic effects may be wanting, the range of formations 
traversed will be greater through the more extensive erosion. Third, 
because in mountainous regions there is an increase of probability 
that mineral deposits will fall in the path of streams, which will effect 
changes in the water, causing abnormal stunting, or extraordinary 
development of given forms. Fourth, because the influx of side 
streams, bearing the water of mineral springs, will add to these effects. 
