THE OUTER TISSUES OF ANIMALS. 311 



covered by slightly-changed dermal tissues. And these 

 arborescent branchiae are gathered together into a single 

 cluster. Thus there is evidence that large external respira 

 tory organs have arisen by degrees from simple skin: as, 

 indeed, they do arise during the development of each indi 

 vidual having them. Just as gradually as in the embryo 

 a simple bud on the integument, 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 branchiae 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 ? Main 

 ly, no 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 

 conditions, should thereby be led to grow into a larger 

 surface of aeration, appears improbable. Survival of those 

 individuals which happen to have this portion of the integu- 



