98 THE RATE OF GROWTH OF FOOD-FISHES. 



the whole of their life, we do not mean by this that they 

 possess the power of unlimited growth any more than that of 

 unlimited life. There is everywhere a maximum size which, as 

 far as our experience goes, is never surpassed. The mosquito 

 never reaches the size of an elephant, nor the elephant that 

 of a whale." 



Limit of Growth. The problem of the growth of fishes is 

 thus very different in many ways from that of the growth of 

 quadrupeds and birds. As one would expect, from theoretical 

 considerations, the fish as an organism shows itself to be more 

 directly susceptible to the influence of its environment. Thus, 

 as mentioned below, the period of incubation can be altered at 

 will between very wide limits by simply varying the tempe- 

 rature, whereas one would hardly expect to alter the period 

 of mammalian gestation, or even of avian incubation, except 

 within extremely narrow limits. Other instances could easily 

 be given to show that the direct effect of the environment 

 upon the piscine organism is much greater than that upon land 

 organisms. Without considering the effect of environment it 

 is probable that the life-cycle of the individual fish also differs 

 in very important respects from that of the higher animals. 



' In the mammalia there is a definite duration of growth 

 in bulk and of life quite apart from the environment, and 

 either period can only be altered by continued action of the 

 environmental factors through many generations. There are 

 some experiments which point to the conclusion that the 

 duration of life of the individual fish is only limited in the 

 widest sense (with immunity from environment), but leaving 

 this out of the question there is no proof of the hypo- 

 thesis that the individual fish ceases to grow at any period 

 of its life ; on the contrary, there are considerations which point 

 to the other view, i.e., that a fish continues to grow throughout 

 its life. 



'Thus, if a mammalian or avian species be subjected to 

 the destroying agency of man, there follows a diminution in 

 numbers; whereas, if a fish be subjected to like conditions a 

 reduction in the size of the individuals is the immediate result. 

 These facts can be explained as follows : — In the former case 



