THE OUTER TISSUES OF ANIMALS. 293 



which, in the EoUs and kinds allied to it, are distributed in 

 rows or clusters all along the back. Instead of these, the 

 Doris has appendages developed into elaborately-branched 

 forms small trees of blood-vessels covered by slightly- 

 changed dermal tissues. And these arborescent branchiae are 

 gathered together into a single cluster. Thus there is 

 evidence that large external respiratory organs have arisen 

 by degrees from simple skin : as, indeed, they do arise during 

 the development of each individual having them. Just as 

 gradually as in the embryo the simple bud on the integu- 

 ment, with its contained vascular loop, passes by secondary 

 buddings into a tree-like growth penetrated everywhere by 

 dividing and sub-dividing blood-vessels ; so gradually has 

 there probably proceeded the differentiation which has turned 

 part of the outer surface into an organ for excreting carbonic 

 acid and absorbing oxygen. 



Certain inferior vertebrate animals present us with a like 

 metamorphosis of tissues. These are the Amphibia. The 

 branchiae here developed from the skin are covered with cel- 

 lular epidermis, not much thinner than that covering the rest 

 of the body. Like it they have their surfaces speckled with 

 pigment -cells ; and are not even conspicuous by their extra 

 vascularity where they are temporary at least. They facili- 

 tate the exchange of gases in scarcely any other way than by 

 affording a larger area of contact with the water, and inter- 

 posing a rather thinner layer of tissue between the water 

 and the blood-vessels. Those very simple branchia3 of the 

 larval Amphibia that have them but for a short time, 

 graduate into the more complex ones of those that have them 

 for a long time or permanently ; showing, as before, the small 

 stages by which this heterogeneity of surface accompanying 

 heterogeneity of function may arise. 



In what way are such differentiations established ? Partly, 

 nc doubt, by natural selection ; but also to some degree, I 

 think, by the inheritance of direct adaptations. That a por- 

 tion of the integument at which aeration is favoured by local 



