APPENDIX. 523 
surface of the water in order to repeat this movement ; thus renew- 
ing the impulse and enabling themselves to continue for a longer 
time their passage through the air. Their changes of direction, 
either to the right and left or in rising and descending, are not due 
to the beating of the wings, that is to say, of the great pectoral fins, 
but simply to an inflexion of the whole surface, in one or the other 
direction, by the contraction of the muscles controlling the action 
of the fin-rays, their pressure against the air determining the move- 
ment. The flying-fish is in fact a living; shuttlecock, capable of 
directing its own course by the bending of its large fins. It probably 
maintains itself in the air until the necessity of breathing compels 
it to return to the water. The motive of its flight seems to me to 
be fear ; for it is always in the immediate neighborhood and in front 
of the vessel that they are seen to rise ; or perhaps at a distance 
when they are pursued by some large fish. Now that I have studied 
their movements, I am better able to appreciate the peculiarities of 
their structure, especially the inequality of the caudal fin. It is per- 
fectly clear that the greater length of the lower lobe of the caudal 
is intended to facilitate the movements by which the whole body is 
thrown out of water and carried through the air ; while the ampli- 
tude of the pectoral fins affords only a support during the passage 
through the lighter medium. Nothing shows more plainly the 
freedom of their movements than the fact that, when the surface 
of the sea is swelling into billows, the flying-fishes may hug its 
inequalities very closely and do not move in a regular curve, first 
ascending from and then descending again to the level of the 
water. Nor do they appear to fall into their natural element, as 
if the power that had impelled them was exhausted ; they seem 
rather to dive voluntarily into the water, sometimes after a very 
short and sometimes after a rather protracted flight, during which 
they may change their direction, as well as the height at which 
they move. 
The most common flying-fishes of the Atlantic belong to the 
genus Exocetus, and are closely allied to our Billfish (Belone). 
