Paleeontological researches have not yet brought to light any definite inter- 

 mediate stages between scales and feathers, but that they must once have 

 exist.-d is shown by the development of these structures (eomp. Fig. 12 and 

 description). 



There is no trace of proper dermal bones (dennostoses) l or 

 calcifications in the skin of Birds, and the glands are reduced to 

 a single mass the uropygial gland : this is situated at the 

 base of the tail (uropygium), and its secretion serves to oil the 

 feathers. 



Epidermic structures, such as claws, spurs, foot-scales, 

 and beak-sheaths, are strongly developed. 



6?l'^llPfe---* r - 



LvJ.V.TV^.\;Vs 



FIG. 13. Six STAGES IN THE DEVELOPMENT OF HAIR. 



Sc, stratum corneum ; SJf, stratum Malpighii ; C, derma ; F, follicle ; Dr, sebaceous 

 gland ; CZ, central, and PZ, peripheral zone of the hair-germ ; HK, hair-knob ; 

 P, beginning of the formation of the hair-papilla ; P l , the same in a later stage 

 of development, when it has become vascular. 



Mammals. The hair-like structures possessed by certain 

 Reptiles and Birds are histologieally quite distinct from the 

 true hairs of Mammals. The possession of hairs characterises 

 Mammals quite as much as feathers distinguish Birds from all 

 other animals. 



1 See note on p. 62. 



