44Q FISHES 



study of fishes as of other animals, that the complicated pheno- 

 mena are not to be explained by any one cut-and-dried formula. 

 On the whole it seems most reasonable at present to conclude 

 that the structure and evolution of fishes are due to the 

 constant interaction of two general causes or processes, spon- 

 taneous variation or the occurrence of " mutations," on the one 

 hand and the influence of the conditions of life on the other. 

 The struggle for existence drives fishes, as other animals, to 

 adopt various habits in order to obtain food, to escape enemies, 

 and to produce offspring ; the actions involved in these habits 

 by " functional stimulation " produce changes of structure, and 

 the results of these changes are adaptations, while at the same 

 time the direct action of the conditions may produce results 

 such as blindness in cave-fishes, and absence of pigment on the 

 lower sides of flat-fishes. On the other hand, the differences 

 which distinguish species, and the larger differences which dis- 

 tinguish the subdivisions of the class from each other, appear 

 to be due in the great majority of cases to spontaneous varia- 

 tions or mutations which have no direct relation that we can 

 perceive either to utility or adaptation, or to the habits or con- 

 ditions of life. 



Note. — In connection with the account of electric organs in fishes mention 

 should be made of Astroscopes y graecum which occurs on the east coast of the 

 United States of America. The organ in this fish is small and situated behind 

 the eye on each side : it seems to be formed by modification of part of the eye 

 muscles. 



