REPORT OF COMMISSIONERS OF INLAND FISHERIES. 55 



ing fish, the abundance of other carnivorous fishes which feed on the 

 young and adult rnackerel, and the existing number of fishes and 

 other organisms which furnish the food supply of the mackerel them- 

 selves. Independently of the others, each of these conditions may 

 vary from year to year about a certain average. Some of these con- 

 ditions will be favorable at times when the remainder will be unfavor- 

 able, but in most years, according to the law of chances, the favorable 

 and unfavorable conditions will nearly counterbalance, so that 

 usually there are nearly the average number of mackerel in existence. 

 But in exceptional j'ears the favorable and unfavorable influences 

 do not offset each other, but one set of conditions is so predominant 

 that either a far greater number of mackerel survive than usual or a 

 much greater number perish. For instance, if a large number of 

 eggs are fertilized and hatched, if a smaller number of enemies are 

 present than usual, if the food supply is unusually abundant, then 

 more mackerel will survive than usual, and if such exceptional con- 

 ditions exist for a series of years, then they will thrive in extra- 

 ordinary abundance. But such an exceptional state of circumstances 

 can not long exist unless the environment as a whole is changing. 

 Each of these conditions, in order to maintain an average, must 

 return to the mean, and then after a longer or shorter time vary to 

 the other side of the mean and become unfavorable. Then, when 

 the sum total of conditions becomes exceptionally unfavorable and 

 remains so for a series of years, a period of exceptional scarcity of 

 mackerel results. Thus the balance of forces swings back and forth 

 about the mean, and an extreme fluctuation in the numbers of 

 mackerel results. 



Such an alteration in the abundance and scarcity of a species, due 

 to a fluctuating combination of favorable and unfavorable condi- 

 tions, is a universal principle throughout the animal and plant world, 

 and is the means by which the numbers of individuals of a species are 

 kept constantly adjusted to the fluctuating conditions in the environ- 

 ment. The mackerel doubtless forms no exception to this general 

 law, and the causes of its variation in numbers must be due to var}^- 



