406 



SCIENCE. 



[N. S. Vol. XX. No. 508. 



most other animals, for the simple reason, it 

 may be presumed, of its burrowing habits." 

 I have found, however, in my own study of 

 moles that if the hair is cut close to the body 

 there is displayed in every region a decided 

 hair slope, showing much individual variation 

 and many whorls, tufts and other phenomena 

 such as Kidd elsewhere considers important. 

 Plainly here there is no correspondence be- 

 tween the external or apparent condition and 

 the true hair direction. An equally careful 

 examination of each long-haired form should 

 be made before any definite statement as to 

 its hair direction is given, since it is possible 

 that even an animal exhibiting superficially 

 the so-called primitive hair slope might show 

 in the direction of the follicles unsuspected 

 deviations from this. 



2. Although one of the data with which 

 Kidd prefaces his discussion is ' that the direc- 

 tion of hair can be modified in the life of an 

 individual animal,' he gives no account of the 

 actual observation of such changes. He says, 

 " This is obviously the case in the head and 

 face of man and may be assumed to be so in 

 lower animals though not easily shown in par- 

 ticular cases." If this datum be true, however, 

 and if the hairy coat of a horse is so exact a 

 ' pedometer ' as Kidd believes it to be, it should 

 be easily possible during the training of a 

 colt for the race-course and its subsequent 

 career to observe actual changes of hair direc- 

 tion, and his conclusions will carry greater 

 weight when such observations have been made. 

 Unfortunately the few unconscious experi- 

 ments which are cited as having been per- 

 formed by man are largely negative in their 

 results. 



3. "With regard to the inheritance of peculi- 

 arities of hair direction, much light would 

 probably be thrown upon the subject by the 

 study of conditions in several successive gener- 

 ations. In the case of rapidly breeding ani- 

 mals (e. g., white rats, which I have found 

 show much variation in hair directions of cer- 

 tain regions) such observations might easily 

 be made, and in many cases three generations, 

 at least, of the human species would be avail- 

 able. 



4. As already indicated, there is no explana- 



tion of the process by which the divergent 

 traction of underlying muscles produces its 

 supposed effect upon hair direction. The 

 dynamic relation between such points of di- 

 vergence and whorls is yet to be explained. 



5. The very stronghold of Kidd's argument 

 seems to be the assumed fact that practically 

 none of the modifications of the primitive hair 

 slope are adaptive. Is it not possible, how- 

 ever, that with a structure so complicated, so 

 manifold in its functions, and, withal, at pres- 

 ent so little known as the skin, even a short 

 hairy covering may possess as yet imperfectly 

 understood functions? Our present state of 

 knowledge of this subject is, at least, not so 

 complete that we are warranted in basing an 

 argument upon the assumption of the lack of 

 function of any particular hair direction. 



Purthermore, even granting that a given hair 

 direction is determined in response to mechan- 

 ical causes, may not this very power of re- 

 sponse on the part of the growing tissue be 

 considered an adaptation and, therefore, of 

 selective value? 



On the other hand, ruling out any possi- 

 bility of a selective process as accounting 

 directly or indirectly for peculiarities of hair 

 direction, Kidd fails to show any reason why 

 these may not be simply spontaneous varia- 

 tions which, not held by a selective process to 

 a definite course of evolution, are running 

 riot, as it were. The very large amount of 

 individual variation which Kidd himself ac- 

 knowledges and which has been described by 

 Voigt for the human species is itself an indi- 

 cation of the plausibility of such an explana- 

 tion. 



G. Kidd's treatment of the entire subject 

 presents certain inconsistencies and discrep- 

 ancies, some of which have already been 

 pointed out. Probably the most serious of 

 these is the lack of harmony between his 

 preliminary statement of the cause of whorls 

 and tufts as due to motor phenomena and his 

 subsequent explanation of many of them as 

 due to external pressure. 



7. Perhaps the most serious fault of the 

 whole work is the arbitrariness with which the 

 lines are drawn distinguishing between the 

 three principles governing hair direction. 



