THE RATE OF GROWTH OF FOOD-FISHES. Ill 



the same lapse of time from the hatching date, then the 

 different rate of growth, due to the different environment, 

 would account for the smaller size. 



The other alternative is to assume an approximately con- 

 stant rate of growth in each case, and an earlier (in time) 

 maturation of the sexual organs. 



Further investigation alone can show whether one has to 

 deal with a case of a hastening of sexual development (paedo- 

 genesis) or a retardation of growth. Petersen is inclined to 

 think that the period of attainment of maturity (roughly three 

 years in the Danish waters) corresponds with that of our east 

 coast ' form ' of plaice, which rather indicates the latter. 



We may perhaps appeal to a similar series of phenomena to 

 account for these ' races ' or ' forms ' as in the case of the effects 

 of over-fishing (see above). If in a given district the increase 

 of natural enemies to the plaice is so great that their influence 

 may have the same injurious effect as that of over-fishing by 

 man, then a small form would result, and this would possibly 

 be assisted in warmer regions by earlier maturity, whereas in 

 districts where the species is comparatively unmolested, and 

 especially when in the colder climes, with, for this reason, a 

 retarded sexuality, a larger race or ' form ' would be evolved. 



This phenomenon of ' forms ' practically resolves itself into a 

 special case of the general law stated in the early part of this 

 chapter, namely, that the mean average size of the species at 

 every stage has been determined under natural selection, and 

 is obviously mutable under changed conditions. 



It is convenient to express the facts of growth-rate by 

 growth-curves. These curves are intended to express the nor- 

 mal rate of increase in length of the species under natural 

 conditions, and we should add, in the particular environment 

 in which the specimens were caught. It is preferable to gauge 

 the size of a fish by its length because this dimension is most 

 readily determined, and in consequence all ' prohibitive ' legis- 

 lation must adopt a standard of length. From a scientific 

 point of view the method, as expressing comparative increase 

 in bulk, is inaccurate, for in comparing the growth- curves we 

 must assume that the length is in all cases an equivalent 



