THE INNER TISSUES OF ANIMALS. 333 



to be digested. Now, among the various waste products 

 continually escaping from the living tissues, some of the 

 more complex ones, not very stable in composition, are likely, 

 if added to the food, to set up changes in it. Such changes 

 may either aid or hinder the preparation of the food for 

 absorption. If an effete matter, making its exit through the 

 wall of the intestine, hinders the digestive process, the en- 

 feeblement and disappearance of individuals in which this 

 happens, will prevent the intestine from becoming the esta- 

 blished place for its exit. While if it aids the digestive 

 process, the intestine will, for converse reasons, become more 

 and more the place to which its exit is limited. Equally 

 manifest is it that if there is one part of this alimentary 

 canal at which, more than at any other part, the favourable 

 effect results, this will become the place of excretion. 



Thus, then, reverting to the case in question, we may 

 understand how a product to be cast out, such as biliverdine, 

 if it either directly or indirectly serves a useful purpose, 

 when poured into a particular part of the intestine, may lead 

 to the formation of a patch of excreting cells on its wall; 

 and once this place of excretion having been established, the 

 development of a liver is simply a question of time and 

 natural selection. 



299. A differentiation of another order occurring in the 

 alimentary canal, is that by which a part of it is developed 

 into a lateral chamber or chambers, through which carbonic 

 acid exhales and oxygen is absorbed. Comparative anatomy 

 and embryology unite in showing that a lung is formed, just 

 as a liver or other appendage of the alimentary canal is 

 formed, by the growth of a hollow bud into the peri-visceral 

 cavity, or space between the alimentary canal and the wall of 

 the body. The interior of this bud is simply a cul-de-sac of 

 the alimentary canal, with the mucous lining of which its 

 own mucous lining is continuous. And the development of 

 this cul-de-sac into an air-chamber, simple or compound, is 



