346 REPORTS OF BERING SEA COMMISSION. 



Birtii-rate and In tliG CRse of the Seal or any other animal the 



death-rate. 



condition of the species as to number must always 

 depend upon the relation of the birthrate to the 

 deathrate. As long as these two are equal the 

 number remains constant, provided, of course, the 

 distribution of deaths among the various ages 

 remains the same. Change the distribution, and 

 there will be a temporary increase or decrease in 

 the total number of the species, according as the 

 deaths are shifted toward the later or earlier 

 part of the animal's existence. Thus, suppose 

 twenty years to be the normal age of the seal: 

 if all deaths occur at the end of twenty years, 

 the total number alive at any one time would be 

 much greater than if the mortality was distrib- 

 uted throughout the whole period. When a cer- 

 tain distribution of this mortality is determined 

 upon, however, the number of individuals living 

 at one time will adjust itself to this distribution 

 and will then remain constant, provided, always, 

 that the distribution of mortality is such as not 

 to affect the number of births. If, in m\j species, 

 it could be determined that no deaths should 

 occur until sometime after the reproductive age 

 had been reached, such a species would increase 

 with great rapidity. With equal certainty, if it 

 were fixed that all deaths should occur before the 

 reproductive age, the species would be shortly 



