
30 
This extract from that part of Dr. Roget’s Bridge- 
water Treatise, which treats of animal physiology, 
is a lucid statement of motion in the fins of fishes; 
but, as our object is not merely to direct attention 
to the motion of these organs, but to their analogy 
to the limbs of quadrupeds, and their difference of 
power and agency with their modifications to their 
several purposes, we therefore go on with a further 
extract describing their constructive adaptation for 
movement in their peculiar element : 
‘* Whatever analogy may exist in the structure of 
the fins of fishes and the feet of quadrupeds, there 
is none in the manner in which they are instrumen- 
tal in effecting progressive motion. The great agent 
by which the fish is impelled forwards is the tail: 
the fins, which correspond to the extremities of land 
animals, are useful chiefly for the purposes of turn- 
ing, stopping, or inclining the body, and for retain- 
ing it in its proper position. The single fins, or 
those which are situated in a vertical plane, passing 
through the axis of the body (the mesial plane.) 
prevent the rolling of the body, while the fish darts 
forward in its course. ‘The fins that are in ‘pairs, 
(that is the pectoral and the ventral fins,) by their 
alternate flexions and extensions, act like oars ; 
while they are capable at the same time of expand- 
ing and of closing the rays, like the opening and 
shutting of a fan, according as their action is re- 
quired tobe effective or thecontrary. All these auxi- 
liary instruments are chiefly serviceable in modify- 
ing the direction and adjusting the variations of force 
