Ryder. I 04o [Oct. 18, 



offered to a flat vertical membrane supported by rays diverging radially 

 from the hypural bones or cartilages below the upturned urostyle or noto- 

 chord. Motion is mainly imparted to the caudal fln by the muscles of the 

 urosome, or by that part of the tail of the fish intervening between the 

 base of the caudal fin. and the anus. The motion of the caudal fin is 

 therefore controlled by the posterior part of the vertebral column and the 

 lateral muscles of the urosome, and not through the morphological axis 

 represented by a dotted line deflected upward and terminating between 

 the letters D and V, so that the mechanical axis, or the axis which con- 

 trols the movements of the whole fin, passes out far below the latter 

 along the dotted line ending at M. The consequences are obvious ; 

 the resistance offered by the water to the motion of such an osse- 

 ous framework of diverging rays is such as to break the median ones 

 square across and those slightly below or above the mechanical axis in a 

 slightly oblique direction, while the long rays at the extreme dorsal and 

 ventral margins of the fin are actually broken across at an angle of nearly 

 45° with their own axis. If any other valid interpretation of the origin 

 of the differences in the direction of the fractures or joints of the flnrays 

 of the caudal fin can be proposed, I should be glad to hear of them. But 

 it is inconceivable that any other can be true. 



While what is regarded as conclusive proof of the modification of hard 

 parts, conformably to the operation of purely physical agencies has been 

 offered above, it still remains to prove that the forms of soft parts are so 

 modified. That this may be done is already jevident from the data in my 

 possession in regard to the modifications entailed upon larval stages which 

 undergo specialized modes of development in the egg or reproductive pass- 

 ages of the female parent. If it can be shown that larval stages are 

 structurally modified by physical agencies, it is tantamount to certain that 

 the adult is not exempt from the influence of such agencies. Consequently 

 the old debate as to the efi'ect of use and disuse, and the interpretation of 

 adaptations and inheritance on the basis laid down by Lamarck just eighty 

 years ago, has not yet been disposed of, nor will it be by the fundamen- 

 tally erroneous methods now almost universally employed by those biolog- 

 ical investigators who take the opposite grounds. 



In Fig. 2, the heavy curved or wavy lines drawn across the outline of 

 the caudal fin show that the breaks, while practically conforming to a di- 

 rection parallel to an ideal vertical line drawn across the whole fin, the 

 individual breaks of the separate adjacent rays change position slightly 

 with respect to siich a vertical. If lines are now drawn through the 

 transverse rows of breaks of the successive rays we obtain three lines 

 symmetrically related to the mechanical axis of the fin. Three of these 

 lines correspond to the three complete transverse lines of breaks or frac- 

 tures, while the fourth is not yet complete, but enough of it is shown to 

 prove that when complete it will conform to those in front of it. These 

 major curved lines to which the lines of fracture of all the caudal rays con- 

 form, also themselves conform approximately to the outlines of the pro- 



