402 



SCIENCE. 



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



considered an adaptation and is attributed to 

 natural selection. 



The second law receives little explanation 

 or discussion aside from a few illustrations. 



It is upon the third law that the attention 

 of the author is concentrated, since, obviously, 

 the facts from which this law is deduced have 

 direct bearing upon Lamarckism. 



Those regions of the body of a mammal 

 which are peculiarly open to contact with 

 opposing surfaces or are under the special 

 influence of certain active habits of a par- 

 ticular animal form are designated as ' critical 

 areas.' They are the regions in which the 

 action of the third law may be seen and they 

 exliibit, to a greater or less extent, deviations 

 from the primitive hair arrangement. Of 

 such areas there are eleven, enumerated as 

 follows : ' the naso-f rental, pectoral, cervical, 

 axillary (post-humeral), inguinal, spinal, ex- 

 tensor surface of the ulna, ventral and lateral 

 surfaces of the abdomen, extensor surface of 

 the thigh, gluteal, and the side of the flank.' 

 The deviations from the primitive hair slope 

 which occur upon these areas are classifled as 

 (1) reversed areas of hair, (2) whorls, feather- 

 ings and crests, (3) tufts. 



A reversal involves a distinct opposition to 

 the primitive hair direction and is attributed 

 to the effect of contact with the ground or 

 with other external opposing surfaces. Re- 

 versal is seen on the snout of the lion, for 

 example, and over a still greater area on the 

 nasal region of the horse and its allies. In 

 these cases the hair, instead of sloping in the 

 supposed primitive direction from the tip of 

 the snout toward the top of the head, as illus- 

 trated by the red deer, slopes toward the snout 

 from a point below the level of the eyes in the 

 lion and between the eyes in the horse. This 

 peculiarity of hair direction is explained as 

 correlated with the angle at which the head 

 is carried, and is attributed to the fact that 

 air and other agents with which the face 

 comes in contact oppose the primitive hair 

 slope by stroking the hair toward the tip of 

 the nose. 



Most mammals spend a large proportion of 

 their time in resting, and to the various posi- 

 tions employed much of the reversal of hair 



direction is due. This contact with the 

 ground or with some portion of the body 

 in the habitual sitting or recumbent position 

 accounts for reversals upon: (1) the pectoral 

 region of practically all mammals except un- 

 gulates, (2) the extensor surface of the uhia 

 in carnivores, certain ungulates and primates, 

 including man, (3) the lateral aspect of the 

 abdomen in nearly all carnivores and un- 

 gulates, and (4) the extensor surface of the 

 thigh in many species. As the reversed hair 

 direction upon the extensor surface of the 

 ulna in man has hitherto been considered an 

 important vestigial character, this new ex- 

 planation of it is of especial interest. 



The second named deviation from the primi- 

 tive hair direction, viz., the whorl, involves 

 the divergence of hair from a point and may 

 be accompanied by the phenomenon of a 

 feathering. In those cases in which the 

 feathering occurs two divergent hair streams 

 lead from the whorl, curve on §ach side into 

 the general hair direction of the region and 

 often terminate sharply by a crest or ridge 

 where the opposing hair stream is met. 

 "Whorls and their attendant phenomena are 

 attributed by Kidd to ' strong, very frequent, 

 divergent muscular action ' in the region over 

 which the whorl occurs; but, although he 

 shows in numerous eases an extremely inter- 

 esting correspondence between the location of 

 the whorl, feathering and crest, and the di- 

 vergent arrangement of the underlying mus- 

 cles, he unfortunately omits to explain the 

 mechanical process by which such divergent 

 muscular action could affect hair direction. 

 We naturally infer that the force exerted is 

 applied to the hair follicle, but if this is the 

 case, and it could hardly be otherwise, the 

 effect of the pull would be to turn the external 

 portion of the shaft, not in the same, but in 

 the opposite direction. It is thus somewhat 

 difficult to see how the action of divergent 

 forces upon the follicles could result in the 

 divergence of the external portions of the 

 hairs, i. e., in the formation of a whorl. 



The most abundant illustrations of the 

 whorl are drawn from the domestic horse, the 

 highly developed locomotive habits of which 

 render certain regions exceedingly subject to 



