THE OUTER TISSUES OP ANIMALS. 315 



bird-like covering to the fish-like covering a transition so 

 gradual that no place can be found where an appreciable 

 break occurs; and if the scale-like appendages are not 

 truly scales yet they exemplify an extreme metamorphosis. 

 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 are certain 

 other alliances of 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." 



Further details concerning these tegumentary structures 

 are not needful for present purposes, and are indeed but in- 

 directly relevant to the subject of physiological development. 

 Here they are of interest to us only by involving the general 

 question "What physical influences have brought them into 

 existence? Still with a view to definite presentation of the 

 problem, it will be well to contemplate the mode of de- 

 velopment common to the most familiar of them. 



Suppose a small pit to be formed on the previously flat 

 skin; and suppose that the growth and casting off of horny 

 cells which goes on over the skin in general, continues to 

 go on at the usual rate over the depressed surface of 

 this pit. Clearly the quantity of horny matter produced 

 within this hollow, will be greater than that produced on a 

 level portion of the skin subtending an equal area of the 

 animal's outside. Suppose such a pit to be deepened 

 until it becomes a small sac. If the exfoliation goes on as 

 before, the result will be that the horny matter, expelled, as 

 it must be, through the mouth of the sac, which now bears 

 a small proportion to the internal surface of the sac, will be 



