CAUDAL FIN 35 
like rays, which prove stronger and more serviceable than 
the heavier radials; it seems more capable of adapting 
the fin for special uses. 
Accordingly, in many forms of recent fishes, notably 
bony fishes, the entire fin is found to become of dermal 
origin; the radio-basals, greatly reduced in number and 
size, extend no further outward than the base of the fin; 
they are usually small and irregular, and are often deeply 
sunken within the body wall. 
After this glimpse at the mode of origin of the vertical 
fins, z.e. dorsals and anals, the history of the final vertical 
fin, the tail, and of the paired fins may next be reviewed. 
The Caudal Fin 
The tail, or caudal fin, is the main organ of aquatic 
propulsion, and it is doubtless on this account that it 
presents so wide a range in its structure and outward 
form. From the earliest times there are found fishes of 
all groups whose tail shapes are tapering (dphycercal, Fig. 
47), unsymmetrical (heterocercal, Figs. 45, 46), or squarely 
truncate (homocercal, Fig. 48), as the mechanical needs 
in swimming may have demanded. 
The following summary of the mode of evolution of 
the caudal fin seems to be warranted by study of fossil 
and embryonic forms. The vertical fin fold of the ances- 
tral fish was probably carried around the body terminal 
and strengthened by constant actinotrichia (Fig. 39 C), a 
condition similar to that (Fig. 44) of an early larval 
stage of living fishes (frotocercy). This caudal structure, 
however, could have proven-of value only in sluggish 
undulatory motion. The functional needs, which gave 
rise to radials anteriorly, have in the tail region produced 
firmer and stouter fin supports. These appear both on the 
