44 INITIATIVE IN EVOLUTION 



altogether their ancient inheritance, beginning their innovation 

 perhaps, with Dryopithecus fontani in the Miocene Age. 



The Dynamics of Hair-Pattern. 



There are a few well-known facts which it is necessary to bear 

 in mind if one is endeavouring to understand the mode of origin 

 and order of the events before us. The hairy coat of a mammal is 

 composed of individual hairs of varying length, colour and thickness, 

 each being rooted in a tiny pit in the skin and growing from a 

 papilla at its base. As the hair grows, its free end is pushed away 

 from the papilla at the rate of one inch in two months. This is 

 the rate in man's hair, and it is probably greater in the case of 

 lower mammals on account of the greater importance and physio- 

 logical activity of their hairy coat than in man's. But one inch 

 in two months is a close enough calculation. Here, then, is a struc- 

 ture which grows throughout the whole life of the animal, and has 

 to dispose itself somehow on the surface of the skin. It does this 

 in the line of least resistance, and to trace this line is the Alpha and 

 Omega of the present inquiry. 



There is a conception of much value in understanding the 

 dynamics of the distribution of hair, and that is to view the hair 

 of mammals as composed of certain streams. As in every illus- 

 tration, this conception may be challenged because of some difference 

 the critic may find between these streams and a stream of fluid. 

 It certainly does not leave its bed as do the component parts of 

 a river, a glacier or molten lava, for the base of the hair is fixed. 

 But it will serve, and is at least not more open to objection than 

 certain useful metaphors in biology as when the genealogy of man 

 and animals is pictured as a tree, or the living things of the earth 

 as a " web of life." It is, then, as streams moving at the rate of one 

 inch in two months in the lines of least resistance that I propose to 

 discuss the animal hair and its diverse patterns and offer no further 

 apology for doing so. Just as in the cases of a stream of water 

 with varying banks and rocks in its course, or a glacier with its 

 mountain -sides and sinuous valleys, or a stream of lava with small 

 projecting surfaces of a mountain, our stream of hair flows on, 

 hindered only by adequate obstructions. 



Yet another conception from the region of metaphor must be 

 mentioned. It is one which will commend itself to every mind 

 which has been steeped in thoughts of warfare for five years. We 

 are all soldiers now ; we think in terms of military affairs. In the 

 case of our hair-streams there are in many regions two forces 

 directly opposed to one another, others in which no struggle has 



