THE INXER TISSUES OF ANIMALS. 321 



is, as jaundice shows us, capable of escaping out of the body 

 through all its surfaces, even in so differentiated a type as the 

 highest mammal ; and in the undifferentiated types we may 

 infer that the facility of escape is nearly the same through 

 all the surfaces. For the gradual localization of its escape 

 at a particular part of the intestinal surface, it is requisite 

 only that either some disadvantage consequent on its escape 

 elsewhere should be avoided, or some advantage due to its 

 effect on digestion should be gained ; and this advantage 

 may be either direct or indirect. It is not necessary that 

 the biliverdine should itself act on the food : it is enough if 

 it aids in the elaboration of other matters, either nutritive or 

 solvent. If its presence causes or furthers the formation of 

 glycogen from other components of the blood ; or if it sets up 

 the complex reactions which generate the biliary acids ; these 

 effects will suffice to establish, as the place of its excretion, 

 the place where these products are useful. And once this 

 place of excretion having been established, the development 

 of a liver is simply a question of time and natural selection. 



Whether in this case, as well as in the cases of the exclu- 

 sively secreting glands formed along the alimentary canal (to 

 which a modification of the foregoing argument is applicable), 

 any tendency to localization results from the immediate action 

 of the local conditions, is an interesting question. It is 

 possible that the contrasts between the intra-vascular and 

 extra- vascular liquids at these places may be a factor in the 

 differentiation, as in a case already dealt with. (§ 292.) 

 But this possibility must be left undiscussed. 



§ 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 



