HABITS AND HAIR OF PRIMATES 111 



that if one made a simple map of these hair streams, and at the 

 side of it a drawing of the platysma myoides muscle, it could not 

 fail to strike one that the correspondence of the surfaces occupied 

 by the two phenomena was very significant. It is going too far 

 to say that the correspondence is complete, but it is so nearly so 

 that one may fairly say that the reversed stream of hair which 

 begins at the second rib and goes up the neck, lies over the platysma 

 muscle. The stream of hair does not extend up to the lower part 

 of the face and lower jaw, it does not cover the outlying portion 

 of the platysma on the side of the neck and it begins on the chest 

 a little above the rather uncertain origin of the platysma fibres 

 from the fascia of the chest. But the correspondence of its surface 

 with the main part, or about five-sixths of the platysma, is most 

 suggestive. 



This muscle is one of the subdermal sheets that are found 

 in many mammals, and though it is not a continuation or descendant 

 of the fly-shaker or panniculus carnosus, which is often referred 

 to in these pages, it is an analogous feature of man. It is closely 

 attached at its lower part to the skin over it and more loosely at 

 its upper. It has various functions attributed to it, as I will 

 mention later; but there is one effect of its action which is very 

 evident in a thin person, that is to say, it wrinkles the skin over it 

 in a vertical direction. This it does, whatever else it may do. 



Struggles of the Platysma. 



In interpreting this novel hair stream of man's chest and 

 neck we are again brought into an atmosphere of struggle of forces. 

 Something has occurred in the course of man's descent from the 

 ape to interfere very sharply with the course of the hair ; and 

 certainly if there be anything in organisms that Heredity, Variation 

 and Selection are unable to do (even when adorned with capital 

 letters, to make them, as Huxley said, " like grenadiers with bear- 

 skins," appear much finer fellows than they are), it is to provide 

 in this reversed stream of hair on man's chest some cunning 

 " adaptation " to his needs. Selection will not serve ; but I think 

 use and habit will. There can be little, if any, doubt that the 

 frequent and active contractions of the platysma muscle in the 

 course of man's life are the efficient cause of the change of arrange- 

 ment of hair from a downward simian to an upward human slope. 

 To this opinion the anatomist will promptly reply : " Ah ! I have 

 thee there, friend Lamarckian ; are there not any number of apes 

 and monkeys that also have an active and efficient platysma ? " 

 Undoubtedly there are, and I give here, through the kindness of 



