THE STURGEONS AND STURGEON INDUSTRIES. 
245 
The efficient portion of the caudal as a propelling flange is therefore in the main 
the wide inferior part supported by the jointed rays. The axial support and muscu- 
lature of the tail are much nearer its dorsal than its ventral border. It results from 
this that the inferior flange or membrane of the tail, with its supporting rays, as it is 
swayed from side to side, is made to act in much the same way as an oar in sculling. 
The development of the wide inferior lobe of the tail has also effected an upward flex- 
ure or adjustment of its axis and musculature which could not, for mechanical reasons, 
have happened if the dorsal flu-fold had been as wide as the ventral one. This view 
seems to be proved by the chorda or axis keeping its perfectly straight primitive posi- 
tion along the center of the spatulate or diphycercal tail of Protopterus and Ceratodus, 
where the dorsal fold, with its rays, is as wide as the ventral one. This view is further 
strengthened by the fact that the tail of the embryonic sturgeon is spatulate or 
diphycercal, with the end of the notochord dividing it into two equally wide dorsal 
and ventral moieties, as shown in Plate V. That is, the tail of the sturgeon, in the 
beginning, is of the same type as that of Ceratodus. The cause of the upward flexure 
of the axis of the tail of Acipenser is, therefore, to be sought in whatever it was that 
induced the absorption or reduction of the dorsal fin-fold and the persistence of the 
ventral or inferior fold. The cause of that variation began to act early in the line of 
the true fishes since we for the first time detect it obviously manifested in Chimcera, one 
of the lowest types where the inferior fold begins to widen, and as a result we have the 
first faint inklings of the heterocercal state, that is, the posterior extremity of the 
notochordal axis is slightly bent upwards. This adumbration of the heterocercal condi- 
tion in Chimwra is pronounced in proportion to the degree to which the difference in 
width between the dorsal and ventral flu-folds of the caudal is developed. This seems 
to be a rule or law of caudal differentiation in fishes. 
The origin of the mechanism in question can not, however, be understood without 
reference to the mode in which its functioual use would continually tend to intensify 
the heterocercal condition, premising, of course, that some antecedent cause tended to 
widen the inferior caudal fold to b^gin with. That antecedent cause we do not know 
nor can we do more than guess what it may have been. ,The conditions under which 
the tail flu is used in life is similar to that of an oar in sculling, and it is quite plain 
that the parallelograms of force which are thus developed bj'^ the interaction of the 
tail moving laterally and alternately in opposite directions against the resistance of 
the surrounding water must bring about a constant tendency to induce an upward 
flexure of its notochordal axis. This effect can actually be imitated by means of a card- 
board model, cut out in the form of a heterocercal tail and vibrated from side to side 
under water. In this manner only can the origin of the heterocercal form of the tail 
of fishes be explained. Natural selection is utterly incompetent to do so, because it 
must first explain the concurrent or simultaneous variation of about six thousand 
species belonging to no less than three independent phyla. If the tendency was all 
in the same direction, in three independent series, composed of a multitude of species, 
it involves the conclusion that there was no selection. The effect is manifestly the 
result of a habit of movement the initiation of which may, in part at least, be ascribed 
to the intelligence of the creature manifesting such a habit. 
The soft or jointed rays of the sturgeon, as well as of all other soft-rayed fishes, owe 
their jointed condition to the interaction between the fins and the resistance offered 
passively by the surrounding water. This conclusion might be demonstrated beyond 
