320 



Dr. II. II. Swinnerton on Changes and 



for 97 individuals varying in length from -I'S to 6*5 mm. and 

 in age from the time of lia'ching to tlie end of the third week. 

 This is prolonged by a dotted line, to give an idea of the 

 course it would have taken had there been no transloca tion 

 later. 



200 



300 



Fio-. 1. 



The next line represents the average position of the fin for 

 198 individuals varying in length from 10'4 to 15'6 mm. 

 The age of these cannot be definitely stated, because they 

 were not reared in the aquarium and individuals of the 

 same age vary so greatly after the first month *. The 

 position of this line relative to the dotted line and its in- 

 creasing divergence from it show that the fin has shifted 

 backwards and is continuing to shift. 



The longest line represents the average position for 211 

 individuals varying in length from 25 to 50 mm., and exhibits 

 the same facts more markedly. 



Figs. 2, 3, and 4 were made from measurements of indi- 

 viduals varying in size from 4 to 7, 10 to 16, and 25 to 50 mm. 

 respectively. The horizontal line rej)resents the distance 

 from fin to snout expressed as a percentage of the total length 

 of the body. The vertical lines give the number of indi- 

 viduals with the corresponding percentage. From these it is 

 seen that the maximum number have their fins 23 per cent, 

 from the snout in the first group, 33 per cent, in the second, 



* For example, of the two oldest specimens which I hatched and kept 

 in the same aquarium for thirty-three days, one measured 10"8 mm. and 

 the other only 6"4 mm. 



