LIFE HISTORIES 347 



dorsal fin forwards takes place before the eye has migrated. 

 Consequently the migrating eye has to pass under the anterior 

 extension of the fin, although its relation to the skull structures 

 is really the same as in other species. In all cases the apparent 

 movement of the eye is due to the distortion of the skull, the 

 two sides of which develop unequally; the bones between 

 the two eyes are the same as in other fishes and the posterior 

 part of the skull remains symmetrical. The development of 

 the flat-fish is an excellent example of the recapitulation in 

 the history of the individual of the evolution of the race ; but 

 it must be observed that the larval flat-fish does not represent 

 the adult ancestor but its larval form ; when the adult structures 

 are developed the fish is already asymmetrical. 



Until recently the growth of fishes was little understood, but 

 in the scientific investigation of fishery problems a good deal of 

 light has been obtained on such questions as the size and age at 

 which the more valuable food-fishes become mature. This ap- 

 plies specially to the plaice and some other flat-fishes. Fishes 

 do not all become mature at the same size in a given species 

 and the question of size was first attacked by ascertaining that 

 of the smallest mature and the largest immature ; thus the limits 

 for female plaice in the North Sea are from thirteen to seventeen 

 inches. The sizes for males are considerably lower. With 

 regard to age there are various methods of investigating the 

 problem. By keeping specimens in aquaria the age can be as- 

 certained ; for example, the present writer showed that some 

 flounders became mature at two years, others at three and four 

 years. The results obtained by experiment however do not 

 necessarily apply to fish in a state of nature. One method 

 which has been used with some success for fishes such as plaice 

 captured from the sea is to measure a great many specimens, 

 when it is found that the largest numbers are grouped round 

 certain sizes while there are comparatively few of intermediate 

 sizes ; the specimens thus fall into groups which evidently cor- 

 respond to the produce of successive annual spawnings, and 

 thus the rate of growth and the age and size at maturity are as- 

 certained. More recently it has been found possible in many 

 cases to ascertain the age of a fish by marks in its own structure, 

 as mentioned below in the case of the eel. This method was 

 first invented by a German investigator in the case of the scales 



