NOTES ON THE HAIR-SLOPE IN MAN. 309 



that the trend of the hair-streams on these is from the cephalic 

 to" the caudal extremities of the head and trunk, and from the 

 proximal to the distal extremities of the limb-segments. 



Before leaving the question of setiology, I would mention the 

 floating opinion among biologists that certain facts as to hair- 

 slope on the human body are due to or adapted for rain-tracks. 

 Wallace, and Komanes following him, speak of the slope on the 

 extensor surface of the forearm being vestigial in man, and in 

 monkeys from whom man has inherited it, having been acquired 

 by adaptation to their habits of sitting with their hands grasp- 

 ing boughs of trees in the midst of forests with tropical rain 

 falling on them. Leonard Hill says roundly : " The fine hairs 

 on the body and limbs of man ^ are arranged as in the 

 monkey, to point in certain directions, so as to shoot off the 

 rain from the body when climbing," — a statement both crude 

 and too sweeping. In passing, I might digress for the purpose 

 of remarking that whether vestigial or not, there is one animal 

 whose hair-slope on this region fully bears out my contention as 

 to Lamarckian factors. I refer to the Orang, with its wealth of 

 long hair on the forearm, which slopes uniformly in a proximal 

 direction from manus to elbow joint, manifestly because of the 

 simple action of gravitation affecting the mass of long hair in 

 the attitude so common to all higher apes. 



There is also a reference to this question of eetiology of hair- 

 slope in an interesting work by Mr E, E. Thompson,' where he 

 says : " The direction of the hair is determined by two laws. 

 First, the necessity of offering the least possible resistance to 

 the air, and to grass, brushwood, and other obstacles, while the 

 animal is in motion. Second, the necessity for running otf the 

 rain, especially when the animal is at rest. The first law gives- 

 a backward, and the second a downward, direction of the hair." 

 As to this reading of the facts, I would suggest that it is much 

 more likely that the truth is the other way — viz., that the 

 backward general slope of hair in animals is produced hy the 

 constant pressure of the forces he indicates, and by one more 

 important which he does not mention, viz., the habit of clean- 

 ing their fur uniformly found in mammals, rather ihoxi produced 



'^ Manual of Physiology, 1899, p. 324. 

 '•^ Art Anatomy of Animals, 1895, p. 5. 



VOL. XXXV. (N.S. vol. XY.) — APRIL 1901. X 



