CH. XII.] ADJUSTMENT, DIBEGT AND INDIBEC T. 67 



due to the cooperation of countless ancestral forces; and 

 such tendencies are now not improperly classified undcT the 

 head of " physiological polarity," — provided that nothing 

 more is meant by "polarity" than the ability of certain 

 special groups of forces to work different structural changes 

 in different directions. So much for the internal adaptive 

 process. But now, as the result of the parallel process of 

 external adaptation, it follows that the forms due to the 

 internal process can remain constant only so long as the 

 environment remains unchanged. If the changes in the 

 environment are too great or too sudden to be equilibrated 

 by changes in the distribution of the system of internal 

 forces, the system is overthrown, and the organism perishes. 

 But if the external changes are moderate and gradual, the 

 adjustment of the organism to them by means of internal 

 changes, must result in that kind of organic variation known 

 as direct adaptation. "We need not be surprised, therefore, 

 by the parallel variations of whole genera of American trees 

 or Malayan butterflies ; nor need we ascribe them, with cer- 

 tain recent writers, to " occult energies " of the metaphysical 

 sort, or to a kind of pantheistic " intelligence " inherent in 

 nature, or to any other agency unrecognizable by science ; 

 since the necessity for such parallel variations, wherever 

 whole groups of organisms are exposed to like environing 

 agencies, is a corollary from the fundamental principles of 

 vital dynamics. 



We are now in a position to amend quite materially the 

 view thus far taken of the causes of organic evolution. 

 Hitherto we have concerned ourselves too exclusively with 

 the selection of variations, omitting to inquire into the cha- 

 racter and mode of origin of the variations selected. But 

 the latter point is no less important than the former. If 

 variations might occur equally in all directions from the 

 average standard, by reason of circumstances so indefinitely 

 compounded as to make them seem fortuitous, then the 



