300 PHYSIOLOGICAL DEVELOPMENT. 



various kinds of clothing hairs, feathers, quills, scales, 

 scutes. 



Headers who are unfamiliar with the extreme modifiability 

 of organic structures, will be startled by the proposition that 

 all of these certainly all of them but the last, respecting 

 which there may be doubts are homologous parts. In- 

 spection of a few cases makes this seemingly-incredible pro- 

 position not simply credible but obviously true. A retrograde 

 metamorphosis from feathers to appendages that are almost 

 scale-like, is well seen in the coat of the Penguin. Carry 

 the eye along the surface of one of these birds, and there is 

 manifest a transition from the bird-like covering to the fish- 

 like covering a transition so gradual that no place can be 

 found where an appreciable break occurs. Less striking 

 perhaps, but scarcely less significant, are the modifications 

 through which we pass from feathers to hairs, on the surfaces 

 of the Ostrich and the Cassowary. The skin of the Porcupine 

 shows us hairs and quills united by a series of intermediate 

 structures, differing from one another inappreciably. Even 

 more remarkable is the extension of this alliance to certain 

 other dermal structures. "It may be taken as certain, I 

 think," says Prof. Huxley, " that the scales, plates, and 

 spines of all fishes are homologous organs ; nor as less so 

 that the tegumentary spines of the Plagiostomes are homo- 

 logous with their teeth, and thence with the teeth of all 

 vertebrata. Again, it appears to me indubitable that the 

 teeth and the hairs are homologous organs." 



The ultimate justification for classing these unlike parts as 

 divergent modifications of the same thing, is the unity in 

 their modes of development. Besides a linking together of 

 them by intermediate structures, as above indicated, there is 

 a linking together by their common origin. To quote again 

 from Prof. Huxley's essay on "Tegumentary Organs": 

 " The Hairs and fyjines of mammals, the Feathers of birds, 

 and the Integumentary Glands, agree in one essential point, 

 that their development is preceded by that of an involution 



