1058 REPORT OF COMMISSIONER OF FISH AND FISHERIES. [78] 



in length and strength, to apparently compensate for this local suppres- 

 sion. Upon further examination we find that the portions which have 

 been retained are situated in the most advantageous positions in respect 

 to their effectiveness as organs of propulsion. This is pre-eminently true 

 of the lower lobe of the caudal, which has been shifted into a position 

 which is lineally in a plane with the vertebral axis. The mechanical selec- 

 tion which is here implied, has doubtless led to reduction, just as use 

 and effort or impacts and strains have led to a similar selection of cer- 

 tain axially situated digits in the limbs of Ungulates, whch have thus 

 been hypertrophied, while the extra-axial digits have been atrophied in 

 consequence of disuse. The reduction of the fin-rays and interspiuous 

 pieces of fishes has, it seems therefore highly probable, followed from 

 the working of the same principle as I have indicated in my discussion 

 of digital reduction. This general principle is confirmed by every mor- 

 phological condition which we may choose to select, even if an extreme 

 one is chosen, such as that presented by Mola, in which a degeneration 

 and complete loss of part of the caudal axis have led to the assumption 

 of the office of the caudal fin by a series of rays which primitively be- 

 longed to the dorsal and anal series. Here the flat, nearly discoidal 

 body, no longer flexible, must develop some new mode of progression. 

 The result is that, as in the above-named genus and the trunk fishes, an 

 extreme modification of the lateral musculature has occurred b}'^ which 

 this no longer subserves the use of flexing the body, but the fins only, to 

 which the entire system of myotomes are subordinated, jiowerful ten- 

 dons passing outwards to be inserted into the bases of the rays of the 

 median, fins. 



The evidence in favor of local degeneration or atrophy accompanied 

 by local hypertrophy, which in some cases involves a great change of 

 function in some part, is complete, and we may assert with confidence 

 that such changes must produce great displacement or rearrangement 

 of homologous parts with relation to each other, so that there may arise 

 in this way forms which diverge in the most extraordinary way from 

 what would be considered the typical structure of a group. 



Development has rung a great many changes in the evolution of 

 the caudal fin, as we may learn if we take even a very hurried 

 survey of the structure of this organ as found in the different groups 

 of fishes. In some the termination of the chorda has sufl'ered but 

 slight modification or abortion; in others a large amount of the 

 chordal axis has suffered degeneration, as in Lepidosteus, and manj^ fos- 

 sil genera. The development of centra sometimes occurs in some of 

 these forms behind the point where the axis is bent upwards, but in 

 others it ceases abruptly at the point of flexure. In the Salmonoids and 

 Nematognathi the development of centra passes beyond the point of flex- 

 ure, but in these there is a point beyond which almost total degeneracy 

 of the caudal axis asserts itself. This degenerate portion is therefore 



