116 STRUCTURE OF THE VERTEBRATES 



and its derivatives their color. The baboons have patches of 

 brilliant scarlet or blue (the combination giving a purplish 

 color). In most mammals the pigment is a dark brown, the 

 amount and position giving the varying shades. In the human 

 hair red is not a true pigment but a soluble dye. A person, there- 

 fore, might be genetically red headed, with the dye obscured 

 by a layer of brownish pigment. 



Apparently too wide conclusions have been drawn as to skin 

 pigment and climate. In general terms the lighter races live in 

 colder climates. But the Eskimos are darker than their white 

 neighbors to the south; the Congo pigmies are lighter than the 

 taller tribes in the mountains ; and the white Sicilians are darker 

 than some of the Negro tribes. It would appear, therefore, that 

 it is inherited character, and the lighter races have migrated 

 away from the points of greatest actinic energy. (Page 329 dis- 

 cusses the effects of natural selection). 



Albinism is a complete lack of pigment and differs genetically 

 from white coat color. Melanism is an excess of pigment. Pig- 

 mentation in the human is a multiple factor in heredity, and 

 inbreeding never gives a simple ratio. 



4. Hair. Hair is one of the distinctive characteristics of the 

 mammals, and from the evidence as to its phylogeny one may 

 conclude that it was first a sensory rather than a protective 

 structure. The long vibrissae, or lip hairs, help corroborate the 

 belief. The hairs originate in the ectoderm of the embryo. A 

 knot of cells pushes downward toward the dermis, the end 

 forming a hollow vesicle. As development proceeds the cord of 

 cells connecting the vesicle with the outer skin becomes tubular, 

 thus making the hair follicle. The vesicle settles against the 

 mesodermal papilla and invaginates around it, in this way com- 

 ing into contact with a nervous and vascular supply. The hair 

 grows by a proliferation of ectodermal cells from the bottom of 

 the follicle, and as growth proceeds the hair is pushed up the fol- 

 licle to the outside. 



Most mammals are covered with hair except for a few areas 

 — as the glans penis, lips, and soles of the feet. Cetacea are an 

 exception to the rule, and the elephant, rhinoceros, and others 

 have very sparse hair. The distribution of hair is typical for 

 the species or race; in many species there are sex differences; 



