104 THE RATE OF GROWTH OF FOOD-FISHES. 



size for cod's eggs was 1"386 mm. and that for haddock's was 

 1"458 mm. These figures show that the two species have eggs 

 so near in average bulk that the difference is probably in- 

 appreciable by the apparatus used by Dannevig. 



This third conclusion is suggestive. Would it not be 

 possible, with exact physical apparatus to show that at a 

 constant temperature the time of incubation is inversely as the 

 mass, or inversely as the relationship of the bulk to superficial 

 area, or in some way to reduce the specific variation in time of 

 incubation to a function of the mere bulk contained within 

 each egg ? 



Passing to the further effect of temperature-variations upon 

 the growth of the older fishes, there are a number of obser- 

 vations which tend to show that fishes as a rule grow very little 

 during the coldest months of the year. 



Pathological. Fishes appear to be very susceptible — as 

 regards their growth — to slight injuries. Small abrasions of 

 the surface of the body seem to have a marked retarding effect 

 upon the growth of the individual. The importance of this will 

 be seen later. 



Lastly, we have to deal with what we may term the ' indi- 

 vidual tendency.' It is an accepted axiom that individuals 

 vary not only in size but in other respects, and there is no 

 reason to hold that the rate of growth forms an exception to 

 this statement. In the same brood marked differences occur in 

 the size both of the eggs and embryos and there is reason 

 to believe that, cwteris paribus, these differences become ac- 

 centuated as time goes on. 



These factors, briefly touched upon, form such a formidable 

 array of tendencies, all active under natural conditions and all 

 working to the same end, namely, diversity of growth, that 

 they might well appear to make the rate of growth of fishes 

 an insoluble problem, but we do not think matters will be 

 so hopeless as this if carefully considered. 



Under natural conditions, all the fishes of a species in a 

 given limited area have the same habits and all are subjected 

 to approximately the same conditions of temperature, the 

 pathological element may be eliminated by selections of 



