MEDIAN FINS 31 
ures, z.¢. as ridges in the direction of the fish’s axis or 
line of motion. 
Fish fins have long been distinguished as vertical (me- 
dian, or unpaired) or lateral (paired), the former function- 
ing both as keel and means of propulsion, the latter as 
accessory and specialized balancing organs. 
Median Fins 
Median fins are unquestionably the older. They exist 
in the simplest condition in those fishes whose axis is long 
and whose motion is undulating. Indeed, the sole swim- 
ming requisite is here the continuous dermal keel which 
passes down the back from the head to the body terminal, 
and extends thence forward on the ventral side. The 
undulatory motion of the body is well transmitted to the 
surrounding medium by the exaggerated undulation of 
this long, waving fin web. This condition was probably 
the ancestral one in the evolution of fishes. It represents 
the simplest metamerism ; it occurs as the adult condition 
_ in the lampreys (p. 57), and as the embryonic or larval 
stage in all fishes, appearing before any traces of paired 
fins are known ; it is even adverse to their specialization : 
should life habits require undulatory motion, paired fins 
must inevitably tend to disappear (eel, p. 173; Cala- 
moichthys, p. 150). 
From this condition the further evolution of the un- 
paired fins may thus be theoretically outlined. 
The primitive continuous dermal fin could have been 
of little value in active movement: its more rapid undu- 
lations could not have greatly increased the rate of motion, 
Since its web, lacking in supports, would not have retained 
its rigidity. As the simplest means of strengthening the 
fin fold, “ actinotrichia” (Ryder), appear to have been early 
