258 - THE HARMONIES OF NATURE. 



in the water as they please ; and it is curious to observe how, 

 alternately expanding or contracting one fin or the other, they 

 gracefully plough the liquid element in every direction. 



It is no less wonderful how perfectly the size and texture of 

 the fins correspond with the habits and necessities of the dif- 

 ferent species of fishes. Those which traverse vast spaces of the 

 ocean are furnished with large and strong fins, to enable them 

 to struggle against swelling waves and rapid currents ; while 

 these organs are soft in the species which confine themselves to 

 more tranquil waters, or habitually reside in greater depths un- 

 troubled by the winds which agitate the surface. 



In the snake-formed fishes, where the whole vertebral column 

 is extremely flexible, and consequently renders the assistance of 

 the fins less necessary, these ancillary organs are reduced in 

 size and number; while in the exocoeti or flying-fishes, the 

 pectoral fins are of so great a length as to be able to carry them 

 like wings a great distance through the air. Thus they frequently 

 escape into another element from the pursuit of the arrowy 

 bonito or the darting dolphin ; and though gulls and frigate- 

 birds may seize them now and then during their aerial flight, 

 yet there can be no doubt that they have good reason to be 

 thankful for the gift of their pinion-fins, without which they never 

 could have maintained themselves on the high seas along with 

 their predaceous pursuers. 



To enable the fish to rise and sink in the water without 

 continued muscular effort, they have been provided with the 

 air-bladder. This hydrostatic apparatus is of various shapes, - 

 but always of sufficient dimensions to contain, when it is dis- 

 tended, as many cubic inches of air as will render the fish spe- 

 cifically lighter than water ; and as the specific gravities of air 

 and water are to each other nearly as 1'815, a small volume 

 is sufficient to render the lesser fishes lighter than the me- 

 dium they inhabit. When they contract this remarkable 

 gas-reservoir, or press out the included air by means of the 

 abdominal muscles, the bulk of the body is diminished, its 

 weight in proportion to the water is increased, and the fish 

 swims easily at a greater depth. The contrary takes place on 

 relaxing the tension of the abdominal muscles, and thus we see 

 fishes rise and fall in their denser element by the application of 

 the same physical law which is made use of by our aeronauts 



