298 MONTGOMERY. [Vou. XII. 
ing from the physiological exertions to restore the correlation, 
should be greater than in species of comparatively restricted 
and uniform habitats which experience neither so many nor so 
great changes of environment. 
Recapitulation. — Organic variation owes its origin indirectly 
to change of environment. For it is necessary for the existence 
of an organism that its organs be correlated physiologically and 
(consequently) morphologically, and no adaptation to its environ- 
ment, a factor which is also necessary for its existence, can be 
brought about until the organs become correlated. Now when 
a change occurs in the environment, this change checks the 
normal action of one or more of the organs, and influences indi- 
rectly the others, so that the correlation of the organs becomes 
temporarily disturbed or interrupted. The degree of disturbance 
in the correlation of the organs, probably stands in a direct 
ratio to the amount of change in the environment ; and accord- 
ing to the statistics given above, the amount of variation cer- 
tainly stands, as a rule, in direct proportion to the complexity 
of the environment. Naturally, when an interruption of the 
correlation occurs, the several organs become to such a degree 
independent of one another as is the extent of its disturbance : 
the greater the disturbance of the correlation of the organs, 
the more autodynamic the individual organs become. In order 
to adapt itself to the new environment, the organism must first 
restore the correlation of its organs. Now the several organs, 
being no longer held in strict restraint by the agency of a com- 
plete correlation, make use of their temporary degree of physi- 
ological independence in order to restore this correlation. Any 
structural changes resulting from the exertions of the compara- 
tively unrestrained (independent or autodynamic) physiological 
forces of the organs, to restore their correlation, are organic 
variations. 
IV. On VARIATION AS A CRITERION OF DEVELOPMENT. 
In the preceding pages I have tried to analyze briefly the 
processes of progressive and regressive development, and from 
a study of the facts of variation in birds, which show that the 
