1SS9.] 



547 



[Ryder. 



the fins when in functional use, and the resistance offered by the surround- 

 ing water in which the animal swims, is based upon a single series of facts 

 observed in the tails of young trout from a little less than, to somewhat 

 more than an inch in length. In young trout that have just completed tlie 

 absorption of their yelk sacks, it is found that the outer rays of the caudal 

 fin are segmented in a direction different from that observed in the median 

 rays, as shown in Fig. 1. This figure shows that the outer or extreme 

 dorsal and ventral rays are fractured obliquely, while the median rays are 

 broken or fractured in an exactly transverse direction. The obliquity of 

 the fractures of the extreme dorsal rays is also exactly the reverse of 

 those of the extreme ventral rays, so that the lines of fracture lie approxi- 

 mately parallel to an imaginary vertical line drawn up and down over the 

 side of the whole fin. 



Fig. 1. 



Now, what is the significance of these facts ? In the first place, that the 

 fractures are real physical breaks which are caused as the consequences of 

 overcoming resistance is shown by the ragged, bruised appearance of the 

 broken ends of the segments, and by the appearance of new breaks be- 

 tween those first formed as the fin-rays become longer and stronger, as 

 the fish grows in size, until as many as two hundred or more may be 

 formed in the course of each of the lateral halves of a single ray. Ob- 

 viously, the only movements which are effective in bringing the tail into 

 use as an organ of propulsion, are the vibratory movements from side to 

 side, witli which every one is familiar who has ever seen a fish swim. In 

 so using the tail the resistance oflered by the water is that which must be 



