438 FISHES 



in other cases leads to the increased development of the muscles 

 and their powers of contraction. It would seem that there is 

 under certain circumstances a special kind of nerve-impulse 

 which is specifically electric and produces electric changes in- 

 stead of contraction. If this were the case the constant repeti- 

 tion of such stimuli might tend to produce the modification of 

 muscular tissue which has taken place in the electric organs of 

 fishes. 



We have considered in the above pages some of the most 

 striking facts concerning the life and structure of fishes, and 

 may add a few general reflections suggested by these facts. It 

 will be seen that while the truth of the general principle of 

 evolution, the mere proposition that fishes like other animals 

 have descended from remote ancestors through innumerable 

 generations with various changes of structure, and wide diver- 

 gences from the original type, is supported by the evidence of 

 fossil remains, comparative anatomy, distribution and develop- 

 ment, yet in seeking the explanation of this evolution in 

 particular cases we are met with all sorts of difficulties. There 

 may be some who think that Darwin's explanation, variation 

 and selection, is sufficient, but there are others who insist on 

 going farther and inquiring what are the causes of the variation 

 and selection. If by selection is meant the advantage of every 

 detail of structure in relation to a particular mode of life, it is 

 impossible at the present day to uphold this view with regard 

 to many of the most important characters by which fish are 

 classified. We may conclude that the air-bladder arose as 

 an advantage in muddy and fresh waters, and perhaps the 

 gill-cover might be useful in these conditions though it is diffi- 

 cult to see how, but no valid reason has been given for the 

 substitution of a bony skeleton, internal and external, for the 

 cartilaginous skeleton and dermal denticles of the primitive 

 shark-type. No satisfactory explanation in terms of utility 

 has been given for the various steps of the transition from 

 the Fringe-finned Ganoid to the ordinary Teleostean, or 

 from the Soft-finned Fishes to the Spiny-finned, or from the 

 shark-type to the Chimasroid. Yet these are the principal dif- 

 ferences by which fish are classified. Again, when we compare 

 closely allied species with one another it is impossible to prove 

 that the differences are differences of adaptation. In most 



