[17] THE EVOLUTION OF THE FINS OF FISHES. 997 



logical effects. The interaction between the Lyriferous type and its 

 environment must have been pretty uniform in its character, because 

 that environmeiit — water — has been constantly about the same as re- 

 spects its density, penetrability, and the resistance it woukl offer to the 

 movements of the fish like organisms immersed in it, so that in form 

 and organization these latter have within certain limits remained pretty 

 constant in their general conformation, widely as they differ in morpho- 

 logical details. Such a uniformity of tendencies shows that the forces 

 competent to effect their initiation have not been haphazard, or fortui- 

 tous, or dependent upon chance variations to favor their operation, but 

 have been fixed by determiuately acting energies, the effects of which 

 the writer has sought to trace. The embryologists have afforded us the 

 clew to the origin of vertebrate bilaterality from structures which have 

 been functional in the lowest Metazoa, and if this bilaterality so in- 

 herited has the significance which it appears to possess in determining 

 further modifications, such as the differentiation of the vertebrate skel- 

 eton and metamerism in general, we are probably not far from a possi- 

 ble solution of most of the problems which confront us in the simple 

 organization of the fishes, which foreshadows the far more com[)licated 

 organizations of the higher types. 



We have seen that the weak, lophocercal tails of young fishes fre- 

 quently did not contain the materials for the complete differentiation 

 of the axial and median distal appendicular parts of the skeleton. Then, 

 as the upper and lower lobes of the caudal fin are developed, the tend- 

 ency to develop the second anal or "lower lobe" of authors to a greater 

 extent than the upper, there has been a tendency to push the tip of the 

 chorda upward, or to push its ventral parietes inward (Alosa) for some 

 distance in front of its hinder end. This hinder extremity then degen- 

 erates, or extensive anchylosis of the imperfect caudal segments of the 

 axis and their appendicular elements supervenes, which aids in carry- 

 ing the degeneracy of the chorda still farther at this point. The princi- 

 ple is the same, as we find upon careful analysis, which obtains in both 

 the most primitive types of fishes and in the early stages of develop- 

 ment whereby the rays become located and developed anteriorly to the 

 end of the chorda, so that what has followed is mainly the result of the 

 acceleration in the development of the lower or ventral part of the me- 

 dian fin-fold, from which what is morphologically a second anal is formed, 

 in a position in which there is no corresponding dorsal lobe developed 

 as its dorsal antitype, in consequence of which there is a lack of balance 

 on either side of the end of the chorda in respect to the mode in which 

 work is done by the tail, especially by the ventral lobe, which then be- 

 comes a kind of oar or sculling organ. In action this ventral lobe, when 

 moved from side to side, encounters resistance from the water, and as 

 it is alternately flexed in an oblique direction to the line of greatest re- 

 sistance, the resultant of the antagonistic forces will pass obliquely 

 ui^wards and across the axis of the tail ; this obliquity of action of the 



