INO: 23] ORGANIC VARIATION. 299 
amount of variation stands in a direct proportion to the amount 
of change in the environment, to advance a theory of the origin 
of variation, —as due directly to the physiological exertions on 
the part of the organism, to restore the correlation of the organs 
which had been disturbed by a change of environment. This 
theory is sustained by the facts given here, and if it be corrob- 
orated by future studies, we may use it as a standpoint from 
which to review the phenomena of organic variation, as offering 
criteria for the study of the processes of development. 
The question often recurs to the biologist engaged in com- 
parative anatomical investigations, why in a certain species a 
particular organ should be structurally variable, which is emi- 
nently stable in allied species. It has, thus far, been the 
method of the biologist to attempt an explanation of this 
variability by reasons derived from his assumptions as to the 
phylogenetic origin of the group, and of the different forms 
comprising it. But may we not, conversely, acquire some 
understanding of the hitherto unknown, or but hypothetically 
conjectured, development and homologies of an organ, by start- 
ing out from the facts of the phenomena of variation themselves? 
This is a line of research inaugurated by Bateson, and which 
may in time afford important results. 
First of all, it is well to recall to mind the two factors neces- 
sary for the existence of the organism: (1) its adaptation to 
the environment, and (2) the correlation of its organs. When 
the environmental forces become more complex in their action, 
the intimacy of the correlation of the organs being more or less 
in a direct proportion to the degree of complexity of these 
external forces, as will be shown later, —then the structural 
development occasioned by such a change of environment must 
be a progressive one, if the organism would maintain its adapta- 
tion tothe environment. But if, on the other hand, the change of 
environment is tending toward a simplification of the previously 
complex action of the environmental forces, then the organism 
must undergo a regressive structural development in order to 
remain in adaptation to the environment. In other words, if 
the environment is becoming more complex, the structure of the 
organism must also become more complex in order to insure its 
