CH. xn.] ADJUSTMENT, DIRECT AND INDIRECT, 65 



internal processes, we see that direct equilibration "v^hich 

 consists in continually arranging all the units of the organism 

 in accordance with their physiological polarities, exemplified 

 alike in heredity and in correlation of growth. On the other 

 hand the dwindling and final evanescence of organs which 

 are disused, is due to the fact that the nutritive material is 

 all needed by the other organs which are in constant use ; 

 and it may accordingly be regarded as an indirect method 

 of preserving the internal equilibrium of the organism. The 

 process of organic evolution may therefore be summarized 

 as follows : 



External 



Direct • . Adaptation, 

 Indirect • Natural Selection. 



(internal * ' \ Correlation of Growth, 



( Indirect . Use and Disuse. 



Here we have a classification of agencies coextensive with 

 our present knowledge of the subject, and sufficiently com- 

 prehensive to include such factors in the problem as may 

 hereafter be discovered. Under one of these four sub-divi- 

 sions every special process concerned in forwarding organic 

 evolution must be included. For since it is admitted on all 

 sides that specific change is due to the necessity for main- 

 taining equilibrium between the organism and the environ- 

 ment, it follows that every process which results in the 

 modification of species must be a process of adjustment 

 or equilibration, either external or internal, direct or in- 

 direct. In the scientific treatment of the problem, there is 

 room for much beside natural selection, but there is no room 

 for occultce vires, or pantheistic intelligences, or for "ten- 

 iencies," save such as may be expressed as the unneutrahzed 

 iurplus of forces acting in a particular direction. 



But we have now done something more than merely to 

 classify the causes of organic evolution. In the act of 

 classif)ang these, we have arrived at the law or formula 



VOL, II. F 



