THEIR GENERAL CHARACTER AND STRUCTURE 15 
stock is maintained, their strange structure, which at first 
sight might seem to be a serious handicap in the struggle 
for life, must be pronounced a successful departure from the 
symmetrical scheme of Nature. 
The fins count for a great deal in determining the affinities 
of genera and species among fishes. 
The two pairs of horizontal fins—the pectoral and the 
ventral—have already been alluded to as the homologues of 
Pectoral and the anterior and posterior limbs of the higher 
ventral fins. vertebrates, and their functions described as princi- 
pally those of balancers, steerers, and regulators of speed. 
They consist of a membrane, generally more or less trans- 
parent, supported and extended by rays. Sometimes this 
membrane of the ventral fins is brilliantly coloured, as in the 
perch and roach. The position of the pectoral fins is con- 
stant, just behind the gill-opening ; but that of the ventral 
fins is very variable. In most British fresh-water fishes they 
are situated far back on the abdomen, as in the Salmon, Carp, 
and Roach Families ; in some they are on the thorax, as in the 
perches ; in others on the jugular region, in advance of the 
pectorals, as in the burbot and miller’s thumb ; in others, 
again, they are wholly absent, as in the eels and lampreys. 
In the determination of genera and species, the number of rays 
in the pectoral fins is held of little account ; but those in the 
ventral fins are of more importance, and, together with the 
rays of the dorsal fin, afford a sure guide to affinity in the 
Acanthopterygii, or Spiny-finned Fishes. 
The unpaired, vertical fins are generally more conspicuous 
than the others. The most elementary form has been described 
Thevertical by Dr. Giinther as “‘a simple fold of the skin sur- 
fins. —_ rounding the extremity of the tail,” and its gradual 
extension forwards along the upper and under surfaces may 
be traced in the development of fishes. Continuous in the 
primitive form of fish, this vertical fin soon became interrupted, 
and ranged itself in three principal masses—the caudal fin at 
