300 HABIT AND INTELLIGENCE. [cnAP. 



acid whicli is formed by slow combustion tlirougliout the 

 body. The means by which the organism effects this 

 change is to bring the blood into virtual contact with the 

 air or water, according as the animal is an air- or a water- 

 breather, over an extensive surface of very thin membrane, 

 through which the exchange takes place spontaneously by 

 that physical process known as the " diffusion of gases." ^ 



If now in one of those animals, such as many of the 

 naked marine Mollusca, in which^ the function of resjDira- 

 tion is discharged by the whole surface of the body, and 

 Possible there is no distinct respiratory organ ; — if in one of these, 

 respiratory I Say, a part of the skin, from any spontaneous variation, 

 organs. becomes thinner than the rest, or more abundantly sup- 

 plied with blood-vessels ; or if the blood under one part 

 of the skin contains a somewhat larger proportion of car- 

 bonic acid, from the waste of the body, than in other 

 places ; any one of these causes will produce a more rapid 

 exchange through that part of the skin than through any 

 'other, between the carbonic acid of the blood and the 

 oxygen of the external medium, and that part will be a 

 rudimentary, or nascent, respiratory organ ; the flow of 

 blood to and through that part will be increased by the 

 chemical unlikeness between venous and arterial blood, in 

 virtue of the law already stated ; and it appears ' tolerably 

 well established, that an increase in the flow of blood 

 through any part will cause it to grow and develop. 



In order to form a complete theory of this subject, it 

 would be necessary to explain, further, why the nascent 

 respiratory organ not only develops, but develops into that 

 structure which is specially needed for the purpose of 

 respiration. This, however, I am not able to do, and I 

 doubt whether it is possible. Unlike the origin of cellular 

 tissue, or the formation of sap-vessels by the union of cells, 

 the present problem is an extremely complex one. 



Whatever may be thought of this speculation (and I 

 only advance it as such), it is consistent with what we 

 know of the development of the branchise of the Mollusca, 



1 Carpenter's Human Physiologj', p. 264. 



