HABITS AND HAIR OF PRIMATES 113 



its progressive development to that of man. There is evidence 

 in the above account of the muscle that a structure is found in 

 monkeys and man which might operate on the overlying streams 

 of hair in any of these animal forms — or might not — in accordance 

 with the conception of struggle between opposing forces which I 

 have kept in view all through this volume. 



It is evident that in all animals below man the platysma 

 has not achieved any victory by its action over the streams of 

 hair on the chest and neck, and to my mind it is equally evident 

 that in the case of man it has carried through a very manifest 

 " turning -movement." It will be objected, quite properly, that 

 this is a matter of opinion, and the pertinent question will be asked, 

 " How do you account for the absence of this reversed hair-pattern 

 in apes and monkeys and its absence in man, both having an 

 efficient platysma muscle ? " 



The essence of a struggle is that it ends with the victory of 

 one adversary over the other, and as the race is not always to the 

 swift nor the battle to the strong, there is of necessity some un- 

 certainty as to the result of any struggle. The factors of time as 

 well as of overwhelming force are required for most of the victories 

 of man over man, and it is not less so in the victories of habit 

 over ancestry in the direction of hair, as I have repeatedly shown. 

 The required time is clearly at one's disposal for this victory, and 

 the " overwhelming force " of habit and use is purely a question 

 of the degree of repetition and the efficiency of the contractions 

 of the platysma, and its greater use in man than in apes and monkeys. 

 The uses to which it was put in the lower forms not having been 

 sufficiently overwhelming for victory, no change in them has been 

 shown. The cumulative effects of the actions of a developing 

 platysma in man, under the guiding influence of his more com- 

 plex habits of life, have turned the scale in favour of the reinforced 

 forces of habit, and the direction of the hair becomes reversed 

 nearly all over the area covering the muscle. 



We must consider all the forces engaged in this struggle for 

 mastery on the neck and chest of man, and remember on one hand 

 the power of the normal slope of hair, the greater difficulty of 

 altering the direction of the thick long hairs of monkeys and apes, 

 and their relatively long resting hours ; and on the other the 

 shorter and finer hairs of man and the increasing efficiency of bis 

 platysma muscle in varied actions. Professor Keith mentions 

 four functions of the platysma : that of depressing the angle of the 

 mouth and lower jaw, helping to flex the head upon the chest, and to 

 empty the laryngeal air-sac, and protecting the deep parts of the neck 



