324 PHYSIOLOGICAL DEVELOPMENT. 



specifically lighter, and the muscular effort needed to keep it 

 from sinking is diminished or, indeed, if the bubble is of 

 the right size, is altogether saved. The contrast between the 

 movements of a Goby, which, after swimming up towards the 

 surface falls rapidly to the bottom on ceasing its exertions, 

 and the movements of a Trout, which remains suspended just 

 balancing itself by slight undulations of its fins, shows how 

 great an economy results from an internal float, to fishes which 

 seek their food in mid-water or at the surface. Hence the 

 habit of swallowing air having been initiated in the way 

 described, we see why natural selection will, in certain fishes, 

 aid modifications of the alimentary canal favouring its 

 lodgment modifications constituting air-sacs. In 



the second place, while from air thus lodged in air-sacs thus 

 developed, the advantage will be that of flotation only if the 

 air is infrequently changed or never changed ; the advantage 

 will be that of supplementary respiration if the air-sacs are 

 from time to time partially emptied and refilled. The re 

 quirements of the animal will determine which of the two 

 functions predominates. Let us glance at the different sets 

 of conditions under which these divergent modifications 

 may be expected to arise. 



The respiratory development is not likely to take place in 

 fishes that inhabit seas or rivers in which the supply of 

 aerated water never fails : there is no obvious reason w r hy 

 the established branchial respiration should be replaced by a 

 pulmonic respiration. Indeed, if a fish s branchial respiration 

 is adequate to its needs, a loss would result from the effort of 

 coming to the surface for air ; especially during those first 

 etages of pulmonic development when the extra aeration 

 achieved was but small. Hence in fishes so circumstanced, 

 the air-chambers arising in the way described would naturally 

 become specialized mainly or wholly into floats. Their con 

 tained air being infrequently changed, no advantage would 

 arise from the development of vascular plexuses over their 

 surfaces ; nothing would be gained by keeping open the com- 



