202 INITIATIVE IN EVOLUTION 



Such an explanation is, of course, the last resort, and all other 

 possible derivations must be disproved before it can be accepted.' ' 

 The physiologist would probably think such an interpretation was 

 the obviously first resort. The same writer discusses at length the 

 homology of an exceedingly rare anomaly among muscles, the 

 extensor ossis metacarpi hallucis, and his desire on the one hand to 

 find a missing parent for Japhet, and his honesty and accuracy on 

 the other hand lead him to say " even when it is present, it cannot 

 be regarded as directly atavistic, since it does not represent a 

 normal mammalian tendency ." And he adds a gentle but remote 

 suggestion — " Brooks certainly describes such a muscle in meno- 

 branchus and hatteria — two rare and remote reptiles ! " But, lawful 

 and necessary though this be, there must be stages on the path of 

 human evolution where such a method must fail and the anatomists 

 can do no more than hold aloof from theory or speculation, with a 

 certain grim enjoyment of the disputes and difficulties of the 

 genealogists. 



Initiative in Muscles. 

 Initiative in the evolution of muscles clearly occurs somewhere 

 in the stem, and behind the formed expression of an altered habit 

 is the integrating action of the nervous system. This will be by 

 some looked at askance as a deus ex machind and reckoned as part 

 of the argument from ignorance in a way which recalls Weismann's 

 scorn of Lamarckian factors in germinal selection. I submit that 

 what he and Osborn call " the unknown factor " of use and habit, 

 arising in response to new stimuli meets as no other proposed 

 suggestion does the formation of new muscles. Given a certain 

 fundamental architecture of skeleton and musculature, such as of 

 primitive vertebrates, one can, without doing violence to any 

 known facts, place the formation of new organs of movement in the 

 following order : — 



1. Neural changes and habits. 



2. Muscular modifications. 



3. Consequent modifications of bone. It carries the question 

 no further to say that these are correlated, however loose may be 

 the meaning of that word that is understood. If the prerogatives 

 of Selection within the germ, of segregation of unit-characters 

 and dominance, and of mutations are not unlimited in the construc- 

 tion of organisms, there still remains a sphere of action for the 

 initiating power of the nervous system. Bones grow and change 

 their form in response to increased or altered muscular action on 

 them, and it is necessary to look back a stage further in the story to 

 the neural changes however produced. There have been abundant 



