NO. 1596. DALMANELLAS OF THE CHEMUNG—WILLIAMS. 37 
Prior to determining what particular part environment or heredity 
may take in determining the characters of organisms record must be 
made of the relations borne by the characters to a change of cond1i- 
tions of environment and to a breaking of continuity of the sequence 
of individuals in the stream of life. 
To ascertain these relations it 1s not sufficient to study living organ- 
isms alone. From living organisms we may learn of the results of 
past adjustments to environment, but not of the steps by which those 
adjustments were attained. The periods of time open to-man’s ob- 
servation are too brief for demonstrating the consequences of current 
changes of environment upon a mass of living organisms. Nor do 
living organisms furnish the evidence necessary to disentangle the 
effects of uniformity of environmental conditions from community 
of descent. In order to distribute the consequences to their specific 
causes it is necessary to ascertain under what conditions each chief 
force acts with uniformity upon the individuals under study. 
The three chief forces concerned may be called Heredity, Environ- 
ment, and Evolution, using these terms to express those, preexisting 
conditions which, being uniform for any genetically related group of 
organisms, may be assumed to produce uniform morphologic effects. 
Heredity will then connote the forces operating in direct genetic 
descent, that is, the control exercised by the ancestry upon the char- 
acters of offspring. Environment will connote the forces and the con- 
trol exercised upon organisms of the same ancestry by particular en- 
vironmental conditions. Evolution will connote the forces and the 
control exercised upon a race of organisms of like ancestry under 
the same environment by continuous reproduction for periods of 
time of geologic significance. 
In the present study the morphologic results of such forces acting 
upon the representatives of a single genus are employed for the pur- 
pose of discovering those forces and their modes of operation. 
It is assumed as a working hypothesis that differences in form, 
among the representatives of a single genus aggregated in a single 
local faunule, may be traced chiefly to hereditary causes; that differ- 
ences associated with areal distribution may be assigned chiefly to 
environmental conditions; and that differences associated with geo- 
logical succession may be assigned chiefly to evolution. 
The reason for thus distributing the hypothetical causes of differ- 
ences is derived from the following considerations: 
First. In the local faunule environmental conditions may be as- 
sumed to have been approximately uniform for all the individuals 
represented in the faunule, and the stage of evolutional progress is: 
also the same for all, hence of the three chief causes of modification 
we are restricted to the operation of the laws of heredity. 
