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 uncertainty 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 complex 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 his 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 when it is tense_--adding the significant comment that “it is usually active in temper”--I presume this to mean bad temper!
Leaving out of account the emptying of the laryngeal air-sac, is it not evident that the remaining three actions of the platysma are very much more exerted in the case of man with all the numerous occupations and movements of his head and neck, in obedience to his higher brain, than in the apes, monkeys and lemurs, endowed with a fitful activity, with fewer and less variable movements of their head, and long, long hours spent in their particular form of meditation?