76 



KERATIN AND KERATINIZATION 



undercoat. Many of the differences between different furs and fleeces is 

 to be found in the relative development of the over- and under-hair. 



When scales and hairs occur together, as on the tails of rodents, the 

 hair group develops in relation to the scale as mentioned above. The fact 

 that, when scales are absent, the hairs still form in groups suggests that the 

 ancestors of existing mammals may have had a scale associated with each 

 trio group and that in the course of evolution the scale has been lost, 

 leaving the hair group to mark the site. The argument is persuasive, 



Fig. 33. Various arrangements of hair follicles illustrating the formation 



of " trio groups " with the suggestion of a relation to an ancestral scale 



distribution. Reproduced from De Meijere through Noback (1951). 



but, when the origin of the hair-scale pattern itself is considered, we are 

 forced further into hypothesis. Two views have been advanced: (1) The 

 early mammals (or pro-mammals) may have been covered with fine 

 scales which diverged into two types, one of which continued to resemble 

 the reptilian form and the other became reduced in size and was converted 

 into hair. It may be significant here that the guard hairs of the primitive 

 platypus terminate in a flat spade resembling a scale. The follicle here at 

 first produces the spade and then turns over to producing a typical hair 

 shaft (Wildman and Hanby, 1938). (2) The hair precursors may have 

 been sense organs on the scales and later moved off into the softer inter- 

 scale regions where they became true hairs and commenced an independent 

 evolution. Alternatively the scale failed to appear, leaving the hair. 



