THE ADAPTATIONS OF FISH 45 



a small compass, and converting the tail into a solid 

 mass of muscle — the main driving power of the fish. 

 And, further, since the animal floats, or is but slightly 

 heavier than its own bulk of fresh water, the vertical 

 strain on its body is reduced to a minimum, and there- 

 fore the vertebrae may remain — as in sharks — gristly 

 or undemarcated. 



The provisions for altering the direction of move- 

 ment are of the nicest adaptation. Since in most 

 fish the muscular arrangement is employed for hori- 

 zontal progression, and gives no great facility for 

 change of depth, a special provision is made to meet 

 this want : an air-bladder under the backbone is 

 developed, the gaseous contents of which can be 

 increased or decreased in amount, and so the relative 

 weight of the fish increased where descent is needed 

 and reduced where ascent is required. To this end 

 also the paired fins are employed, as well as for turning 

 and steering ; and as the use of these two pairs of 

 fins incurs difficulties of the same order as those already 

 seen in insects, we find in fish, as amongst insects, 

 a tendency to reduce one pair — the hind fins — and 

 to give them another office, whilst at the same time 

 enlarging the fore-fins in accordance with the increased 

 work of guidance thrown upon them ; to which work 

 must be added the maintenance of the unstable 

 equilibrium, over which they keep constant guard. 



The vertically compressed shape of a well- 

 built, active fish is such that it can never remain 

 upright and still. It is ceaselessly making small 



