THE RATE OF GROWTH OF FOOD-FISHES. 105 



specimens, and the 'individual tendency' has a correcting 

 factor in the action of natural selection. Thus the mean 

 average size of the species at every stage has been determined 

 under natural selection, as that which has the greatest im- 

 munity from destructive tendencies of every sort, and greatest 

 chances of surviving ; and the same destructive tendencies still 

 act upon every generation to weed out the maxima and the 

 minima to the preservation of the mean, so that after the early 

 stages the largest or smallest are continually removed, the 

 mean only surviving. This fact is important, for it follows 

 from the same, that if one takes a number of fish-eggs or young 

 larvae, already varying in slight degree as regards size, and places 

 them in tanks in artificial conditions — such as easy access to 

 food, and immunity from actual foes ; then, all the slight varia- 

 tions in size tend to be emphasised, and in course of time fishes 

 of very various sizes will result'. 



This does not come under the head of a theoretical hypothesis 

 except to those, a few of whom may yet be found, who will not 

 acknowledge the principles of natural selection, the struggle 

 for existence, and the survival of the fittest. 



In natural conditions, especially in gregarious fishes, the 

 environmental factors of temperature, nutrition, &c., must be 

 very closely similar, if not identical, for all the individuals of 

 one brood, so that, taking this into account, and also what has 

 been stated about the variation, there are good grounds for 

 supposing that the rate of growth is closely similar for each 

 individual, and that a mean average length for any given 

 species at a given age is a fixed and determinable quantity. 

 The greater the difference in environment the greater the dif- 

 ference in the mean average, till ' forms ' may become marked. 



Thus, in estimating the rate of growth, the great effect of 

 marks or abrasions upon the fish precludes the employment 

 of this method of procedure, although apart from this drawback 

 it enables us to study the growth -rate of individuals under 

 their actual conditions. 



The method of rearing young fish in confinement and 

 measuring them periodically must be condemned once for all 

 1 A. T. M., loc. cit. 



