228 The American Geologist. October, i-joi. 
in men, a function of organization? Or is all evolution con- 
ditioned upon physical circumstances. In n^any ways it must 
be, and perha]) this long quiescence of potential variability 
in the lamellibranchs proves it. 
At any rate we are afforded a g-limpse at a cause for varia- 
tion in the succession, shown in the New York jjalaeontolog- 
ical studies, of the Devonian bivalves. It is this. 
The recession of the sea causing the shallow water forma- 
tions of the Marcellus, Genessee, Portage. Catskill, was al- 
ways succeeded in the Hamilton, Chemung, and probably 
Waverly, by an increase of species. It would then appear 
probable that the expelled genera in each instance, driven to 
more restricted areas, probably in deeper water, came into 
some sort of competition, and species, as fixed variations, were 
stimulated to appear, and in the next encroachment landward 
nuiltiplied. Comparing the Chemung with the Hamilton the 
genera prominently exhibiting this expansion are Pterinea, 
Antinopteria, Lcptodcsuia, Glossitcs. Myfilarca, with the new 
genera Ci'cnipectcn, PiycJioptcria Edmondia. These genera 
and their species give some evidence of deeper water origin 
in their smaller size. Naturally this statement cannot be made 
into a sweeping deduction, but it has a preponderant appli- 
cation. The large sized genera Liinopteria GJyptodcsina are 
Hamilton, the small sized genera Crcnipcctcn Eduiondia are 
Chemung, and throughout, with many exceptions, the impres- 
sion of the Chemung species is that of smaller sized individ- 
uals. The voyage of the Challenger lends support to this 
view and Sir Wyville Thompson has himself said "ni}- present 
impression is that although life is thus universally extended, 
the number of species and of individuals diminishes after a 
certain depth is reached, and that at the same time their size 
usually decreases." 
If the increase of species has resulted from competition in 
circumscribed sections of the sea-bottom, this crowtled condi- 
tion may have had also some inliuence on size. 
The assumption of recurrent periods of competition with 
the consequent multiplication of species introduces what has 
always seemed a difificult inquiry, how the process of the sitr- 
z'ival of the fittest works amongst such organisms as bivalve 
.shells, for the most part, sessile sightless animals ; how ad- 
