of the Fishery Board for Scotland. 



291 



than the female. This is undeniably the case in Myxine, and also in 

 the salmon. Thus we may assume that the rate of growth in the two 

 sexes is closely similar. Both sexes are placed under the same environ- 

 mental conditions, and hence a different rate of growth would be at least 

 improbable, whereas the more rapid maturation of the male elements may 

 be traced to a deep-seated origin.* 



Factors of Environment. 



Nutrition. — There is no doubt that the amount of nutrition is a factor 

 coming into play on the assumption of the post-larval stage, which has a 

 direct and marked effect upon the rate of growth. One need not do 

 more than recall the fact. 



Temperature. — -The direct effect of temperature upon the duration of 

 the hatching period in pelagic eggs is a well-known fact. Nearly all the 

 common eggs hatched in the St Andrews Laboratory have been observed 

 to have a shorter or longer period of incubation, according to the temper- 

 ature of the water. 



There are also facts to show that fish grow very little during the colder 

 months of the year, so that the growth-rate must be rhythmic in the 

 natural state. 



Pathological. — Experiments with marked fish have shown that a slight 

 abrasion of the surface may have a remarkable effect in retarding their 

 growth. 



Lastly, there is the important fact of an inherent tendency in each 

 individual to grow at a greater or less rate than its companions. Varia- 

 tion in size of the eggs and contained larvae of fish, such as the plaice, is 

 a patent fact, and may account for a great deal of diversity of size in fish 

 of an equal age. At the same time, it is probable that this factor, in 

 natural surroundings, has been largely over-estimated. The mean average 

 size of the species at every stage has been determined under natural 

 selection, as that which has greatest immunity from destructive tenden- 

 cies 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 exceedingly 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 place them in artificial conditions 

 — such as easy access to food, immunity from actual foes, &c, — in tanks j 

 then, all the slight variations in size tend to be emphasised, and in course 

 of time fish of very various sizes will result. This is precisely what has 

 been done recently by Cunningham.! He took roughly from " 200-300 " 

 young flounders, varying in size from 1*15 to 1*2 cms. in length, and 

 confined them in tanks (of different sizes). At the expiration of a year 

 he found some to be 1*6 inch, whilst others were 7*5 inch. Cunningham 

 has no doubt proved by this experiment the well-known fact that a 

 species tends to vary in size as well as in structure, but so far as we 

 can see, nothing else. 



He points out himself that his data do not apply to natural conditions, 

 but nevertheless proceeds to assign ages to caught specimens, which 



* See 'Hermaphroditism in Cod.' A.T.M. 

 t Jour, Marine Biol. Assoc., 1890-91. 



