226 PROCEEDINGS OF THE ACADEMY OF [Mch., 
The range of individual variation in Ashmunella among specimens 
from one place is (with the exception of A. 1. heterodonta) not greater 
than in Polygyra. Among specimens I have measured or examined, 
I have seen no lot which would yield a markedly bimodal curve were 
the variations plotted. The variations betwcen different colonies or 
gens are often appreciable, sometimes conspicuous; but here also the 
case may readily be paralleled in Polygyra, although usually not in 
such restricted areas, for the reason that in the Polygyra country the 
topographic and climatic features are less emphatic, and the life-zones 
are not crowded upon one another as in the land of the Ashmunellas, 
but are spread over larger arcas. 
The conception of species in such sedentary animals as snails is far 
from simple. A “species” comprises a multitude of colonies or com- 
munities which at any one time are isolated one from the other by the 
existing topographic and other surface features of the country. This 
is and always has been the case, even with the common, widespread 
forms of the more level parts of the country; but the colonies there have 
always been subject to frequent mixture with their neighboring colo- 
nies, by the ever slightly fluctuating conditions of woodland and local 
moisture, so that their network over the country has been here and 
there made practically complete within comparatively short periods. 
As a consequence, we have in many cases no tangible difference 
between individuals from colonies hundreds of miles apart. 
In regions where the local physical features are more accentuated, 
the colonies or communities are often less subject to mixture. More- 
over, the range of conditions within a limited area is far greater. Thus 
snails of the same original stock living in the rocky talus on opposite 
sides of a canyon are often subject to very diverse conditions of heat, 
moisture and consequently cryptogamic food. They are often wholly 
unable to cross from one side to the other by reason of a wide, freshet- 
swept or arid space. Moreover, subsequent changes, such as the for- 
mation of lateral canyons and the localization of suitable stations in 
the talus, tend to further isolate the several colonies, and to preserve 
their individuality for long periods. 
Thus each colony follows its own bent; and differentiation ensues, 
cither by the cumulation of organic changes induced by varying 
conditions of growth and nutrition, determined by the local 
environment as mentioned above, or by the occurrence of diverse 
“mutations” in the several colonies, or by both causes. My idea of the 
practical isolation of snail colonies is bascd upon the experience of 
“many years. Similar views have been expressed by Hemphill, in the 
