164 HISTORY OF THE HUMAN BODY 



dorsal (one or more) and the anal function as keels to re- 

 tain the equilibrium and prevent the body from rolling side- 

 ways, and the caudal fin is the main organ of propulsion, but 

 may act in part also as a rudder to regulate the direction in 

 which the animal moves. The paired fins, which bear the in- 

 appropriate names of pectorals and ventrals, act as subsidiary 

 oars or paddles and seem mainly to guide and maintain the 

 course. 



A recent suggestion, which serves as an addition to the fin- 

 fold theory, is given in Fig. 42, c,.in which it is supposed that 

 the lateral folds are primarily distinct from the median one, 

 and that the paired fins develop from their free extremities, 

 where the stress of motion naturally comes. This furnishes 

 a reason other than chance for the formation of two, and only 

 two, pairs of fins, and also explains a sort of symmetry shown 

 in the two sets of fins of many fishes, since the free edge, 

 and hence, the strongest development, is anterior in the for- 

 ward fin and posterior in the back one. 



The median fins, being of use only in the water, disappear 

 above the fish, although the necessity of similar organs for 

 aquatic life is well shown by the fact that in secondarily aquatic 

 higher vertebrates which have returned to the water although 

 derived from a terrestrial ancestry, some new form of median 

 fins becomes developed. Such animals have lost the serviceable 

 median fins of fishes, with their strong skeletal spines, and, as 

 they cannot recall them, are forced to develop some make- 

 shift arrangement to serve the purpose. Thus, aquatic am- 

 phibians develop a tail fin of integument without skeletal sup- 

 port, the tail of the crocodile is supplied with keels formed of 

 projecting scales, and the whale has manufactured perfect 

 dorsal and caudal fins out of whole cloth, as it were, since they 

 are made from thick, though hairless, mammalian integument, 

 reinforced by connective tissue. The dorsal fin of this latter, 

 as is the case with other external details, is wonderfully fish- 

 like, but the caudal fin is flattened the other way, and extends 

 laterally, instead of up and down, as in fishes. 



To summarize, then, the original median fins of fishes have 



