THE 



AMERICAN NATURALIST 



Vol. XLIV April, 1910 No. 520 



THE OEIGIN OF THE ELECTRICITY TISSTKS 

 IN FISHES 1 



PROFESSOR ULRIC DAHLGREN 



Princeton University 



Among the many specializations of animal tissues four 

 are so fundamental in nature and so specific in their func- 

 tion that they stand out as exceptionally favorable objects 

 for study, particularly as to their origin and evolution. 

 These tissues are those which produce motion, heat, light 

 and electricity in quantities, and for the benefit of the en- 

 tire organism. 



Of these, motion is the most important. Without the 

 power to move it is probable that few animals would be 

 able to survive and to evolve to any great degree of spe- 

 cialization. Thus the forms which did not develop organs 

 of motion at an early period or those which lost it sub- 

 sequently would be eliminated and would leave all other 

 and higher degrees of specialization to be attained by 

 such animals as had developed or retained muscle tissue. 



Heat production was probably an important factor in 

 the development of many land forms, especially in regions 

 of the earth that became subject to cold, and the posses- 

 sion of any small part of this power would tell to a 

 marked degree in the favorable selection of its possessor. 



