1S80.] 



549 



[Ryder. 



files of the successive myotonics, or lateral or muscular segments of the uro- 

 some when viewed from the side. These further correspondences and 

 curves are not insignificant. They are undoubtedly to be traced to the 



Fig. 3. 



properties of strength at different points of the material fractured and the 

 way in which the energy of motion exhibited by the myotomes of the 

 lateral muscles of the tail is exerted upon rays lying at different levels in 

 the caudal tin. 



It may be stated that these figures are from actual camera drawings of 

 the objects themselves, and that no liberties whatever have been taken in 

 recording the facts, as permanent preparations in my possession will tes- 

 tify. 



A study of the other fins of young fishes discloses the fact that the soft 

 rays in the dorsal, anal, pectorals and ventrals are fractured in conformity 

 with the exigencies of use. A study of the types of orders shows that 

 wherever the " soft rays " occur their transverse fractures are due to the 

 same cause ; sometimes even pseudo-arthrodial articulations may thus re- 

 sult. The final conclusion is that the fractures of the "soft rays" of the fins 

 of some six thousand species of fishes now living are the direct results of 

 use. Disuse of the fins would result in absolutely depriving all this host 

 of forms of one of their most salient characters. Whether the effects thus 

 mechanically produced anew in the course of the life of every generation 

 are inherited, is a matter of no consequence, since there is no need in this 

 case for an appeal to the influence of heredity. 



