1392.] The Contemporary Evolution of Man. 479 



tion for several generations and then goes back without any 

 warning to a function which it had thousands of years ago. 

 Thus the force of reversion strikes us as a universal factor. 



Now the singular fact about reversion is the frequent proof 

 it affords of what Galton has called " particulate inheritance. 1 ' 

 When the extensor indicis reverts all the muscles around it 

 may be normal ; therefore we are obliged to consider each <>f 

 these muscles as a structure by itself with its own particular 

 history and its own tendencies to develop or degenerate 1 bus 

 it is misleading to base our theory of evolution and heredity 

 solely upon entire organs; in the hand and foot we have 

 numerous cases of muscles in close contiguity, one steadily 

 developing, the other steadily degenerating. Reversion very 

 rarely acts upon many structures at once; when it does we 

 have a case of diffused anomaly, some repetition in the epi- 

 dermis or in the entire organism of a lower type. Yet in spite 

 of reversion and the strong force of repetition in inheritance, 

 the human race is steadily evolving into a new type. We 

 must, it seems to me, admit that an active principle is con- 

 stantly operating upon these particular structures, guiding 

 them into new lines of adaptation, acting upon widely sepa- 

 rate minor parts or causing two parts, side by side, to evolve 

 in opposite directions, one toward degeneration the other 

 toward development. 



I may now recall the two opposed theories as to what this 

 active principle is : 



The first, and oldest, is that individual adaptation, or the 

 tendencies established by use and disuse upon particular struc- 

 tures in the parent are, in some degree, transmitted to the off- 

 spring, and thus guide the main course of variation and adap- 

 tation. ... , 



The second is that all parts of the body are variable, and 

 that wherever variations take a direction favorable (that is 

 adaptive) to the survival of the parent they tend to be pre- 

 served ; where they take the opposite direction they tend to be 

 eliminated. Thus, in the long run, adaptive variations are 

 accumulated and a new type is evolved. 



