THE IICNER TISSUES OF AMMALS. 323 



no difficulty in understanding how, by infinitesimal steps, 

 they pass into complex and ever-moving lungs. But 

 how is the differentiation that produces them initiated ? 

 How comes a portion of the internal surface to be specialized 

 for converse with a medium to which it is not naturally 

 exposed? The problem appears a difficult one; but there is 

 a not unsatisfactory solution of it. 



When many gold-fish are kept in a small aquarium, as 

 with thoughtless cruelty they frequently are, they swirn 

 close to the surface, so as to breathe that water which is from 

 instant to instant absorbing fresh oxj^gen. In doing this 

 they often put their mouths partly above the surface, so that 

 in closing them they take in bubbles of air; and sometimes 

 they may be seen to continue doing this the relief due to 

 the slight extra aeration of blood so secured, being the 

 stimulus to continue. Air thus taken in may be detained. 

 If a fish that has taken in a bubble turns its head down- 

 wards, the bubble will ascend to the back of its mouth, and 

 there lodge; and coming within reach of the contractions of 

 the oesophagus, it may be swallowed. If, then, among fish 

 thus naturally led upon occasion to take in air-bubbles, there 

 are any having slight differences in the alimentary canal that 

 facilitate lodgment of the air, or slight nervous differences 

 such as in human beings cause an accidental action to be- 

 come " a trick," it must happen that if an advantage accrues 

 from the habitual detention of air-bubbles, those individuals 

 most apt to detain them, will, other things equal, be more 

 likely than the rest to survive ; and by the survival of 

 descendants inheriting their peculiarities in the greatest 

 degrees, and increasing them, an established structure and an 

 esta.blisb.ed habit may arise. And that they do in some 

 way arise we have proof : the common Loach is well known 

 to swallow air, which it afterwards discharges loaded with 

 carbonic acid. 



From air thus swallowed the advantages that may be 

 derived are of two kinds. In the first place, the fish is made 



