294 PHYSIOLOGICAL DEVELOPMENT. 



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- 

 ment somewhat more developed, seems here the only likely 

 cause. Nevertheless there is reason for suspecting that 

 respiratory activity itself aids in the development of a re- 

 spiratory appendage. The reason is this. Exchange of liquids 

 through membrane depends on some difference, physical or 

 chemical, between the liquids : if they are in all respects 

 ' alike, and under equal pressures, no exchange will take place ; 

 while, conversely, if they are* much unlike there will be a 

 rapid exchange. Now through the walls of capillaries, or 

 through the sides of lacunas not yet developed into capillaries, 

 there continually goes on an oozing both ways from the 

 blood into the tissues and from the tissues into the blood. 

 By this double movement nutrition and depuration are alike 

 made possible ; and it is obvious both that in the absence of 

 difference it would not occur, and that nothing would be 

 gained if it did occur. Among other differences continually 

 arising between the intra- vascular liquid and the extra- 

 vascular liquid, is that due to their unlike charges of 

 oxygen and carbonic acid. This difference, like other differ- 

 ences, will cause exchange the rapidity of the exchange 

 doubtless being greater where the difference is greater. 

 Hence if any part of an aquatic animal's skin is nearest to 

 the place where the blood has become most highly carbonized, 

 or if it is so bathed with moving water that the plasma 

 beneath its surface is more oxygenated than elsewhere, or 

 both ; then, other things equal, this part of the skin will be 

 the seat of an osmotic movement greater than goes on in the 

 rest of the skin. But the exchange of oxygen for carbonic 

 acid, proceeding faster here than elsewhere, will have for its 

 accompaniment a more rapid exudation of nutritive matters. 

 The liquid passing out of the blood-vessels to be replaced by 

 the liquid passing into them, is a liquid containing the 

 substances that build up the surrounding tissues. Hence 



