106 THE RATE OF GROWTH OF FOOD-FISHES. 



as absolutely useless ; for, although the temperature may be 

 registered, the conditions of nutrition and the exclusion of the 

 check caused by natural selection upon ' individual tendencies ' 

 make all results obtained in this way abnormal and useless. 



This method has been resorted to by Mr Cunningham at 

 Plymouth and by some other workers. Although we have 

 under the heading of each species quoted some of his obser- 

 vations by this method, yet from what we have remarked 

 above, it will be manifest that there are strong objections 

 (which cannot be lightly dismissed as being 'theoretical') to 

 conclusions deduced from such experiments. 



This being the case the growth of fishes under natural 

 conditions must be studied in another way. 



If a great number of fishes be caught and measured and the 

 date of capture registered, and again, if the spawning-period 

 of the species be known, then the age of each fish may be 

 estimated with a fair degree of accuracy. A number, the more 

 the better, may be caught and measured at one elate and the 

 mean size determined ; we may then assume that the indi- 

 viduals of mean size at any date correspond in age with the 

 mean spawning period, or the period in the middle of the 

 spawning-time at which there are always the greatest number 

 of eggs laid. In doing this we assume that the greater part of 

 the diversity in size of one ' haul ' of fishes, if they are within a 

 year of each other in age, is due to the extended spawning- 

 period ; in fact, is due to a greater or lesser age. A great 

 number of facts and observations tend to bear out this hypo- 

 thesis. The method is probably capable of great accuracy, 

 provided a sufficient number of specimens are caught and 

 measured and the true mean for the district thereby deter- 

 mined, but approximate results may be obtained from even a 

 few specimens, by gauging the age of each individual, within 

 the limits allowed by the spawning-period, as being that which 

 best corresponds with that of a different age. If the age of 

 each fish be expressed by a point upon a curve then each is so 

 estimated as to make the curve as regular as possible. Here 

 we assume that the individual fishes are derived from eggs 

 spawned at different times. The method is capable of a fair 



