158 Cincinnati Society of Natural History. 
Fifth, because here we have the maximum of extremes in rate of 
current, and consequently the maximum of capacity to transport sedi- 
ments that may act favorably or unfavorably upon the various 
creatures inhabiting these streams. Sixth, because of the probability 
that these mollusks have been propagated down stream, to the limit 
of favorable conditions—-a limit always determined in the first place 
by geological causes—-and because of the variation in the conditions 
met in this traverse. Seventh, because combined with all these causes 
is the fact, that all the stages in the development of these creatures are 
passed in an element thus unstable, amid conditions thus diversified, 
where the slightest tendency to variation must have the maximum of 
exciting causes constantly operating to call it into play. If, then, it 
be admitted that there is in the animal races any capacity for adapta- 
tion, and any tendency to variation, life, under such circumstances, 
would be a continuous development and exercise of these inherent 
qualities. For mountain regions have been the seat of origin of all 
drainage, and, no doubt, of the first forms of life inhabiting that 
drainage. 
Now let us examine these probabilities in the light of the actual 
facts connected with the distribution of certain fresh water-shells. 
First, we may consider the circumpolar distribution of certain Zim- 
neide. ‘These mollusks are essentially lacustrine, for while they are 
distributed into rivers and smaller streams to some extent, their sta- 
tion of fullest development is in lakes the world over. 
The genera, Physa, Limnea and Planorbis, are essentially northern 
forms, for it is in the cooler regions of the earth that they reach their 
largest size and greatest differentiation. Distribution southward is 
accompanied by a stunting of forms, in all cases but that of the sub- 
genus Bulinus, of which the B. aurantium passes through the Amer- 
ican tropics, and is many times the size of its cireumpolar northern 
relative, the well-known B. hypnorum. This case stands as the only 
exception to an otherwise universal rule, in a group of mollusks cover- 
ing in many described species, and yet one in which the differentiation 
of forms has led to such interminable varieties, that the most critically 
accurate of our conchologists hesitate to label them. The careful 
student of our North American forms, will find these shells more 
closely allied to their European relatives than any other group of 
mollusks found on the two continents, unless it be the Succinine, and 
a few littural marine species; and as it is not possible to separate the 
species, inter se, upon snatomical distinctions, in the greater number 
‘ 
