REIL Ee i Ee oe ee Ae ey Peer ea esse Y 
and the probable causes of their Variations. 205 
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 these circumstances 
is the fact, that all the stages in the development of these 
creatures are passed in an element thus unstable, amid condi- 
tions thus diversified, where the slightest tendency to varia- 
tion must have the maximum of exciting causes constantly 
operating to call it into play. If, then, it be admitted that 
there is in t i 
water shells. 
First, we may consider the circumpolar distribution of the 
Limneide. These mollusks are essentially lacustrine, for while 
they are distributed into rivers and smaller streams to some 
extent, their station of fullest development is in lakes, the world 
over. 
he 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. Dis- 
tribution southward is accompanied by a stunting of forms in 
all cases but that of the sub-genus Bulimus, of which the B. 
aurantium passes through the American tropics, and is many | 
times the size of its circumpolar northern relative, the well- 
known B. hypnorum. This case stands as the only exception 
most critically accurate of our conchologists hesitate to label 
the The careful student of our North American forms will 
lons at north ; second, their cireumpolar distribution ; 
third, their presence in regions unfavorable to the development 
of other families of mollusks, as testified by their absence; 
b] 
‘fourth, their persistent appearance together, even south ward, 
