THE E^>UILIl!iaUM TIIEOKY. 157 



THE EQUILIBRIUM A THEORETICAL FACT. 



There i.s, however, a certain amount of truth in this idea of ei|uilibriuni, and we 

 may iuqiure what it is and what will be the condition of the herd and of the industry 

 of pelagic sealiiii; when it is reached. 



As already indicated, tiie condition of the fur-seal herd is determined by the 

 relation of its various losses to its single source of gain, the yearly accession of young 

 3-year-old breeders. I'^roni the history of the herd in the period I'rom 1871 to IS.SO 

 we know that the various losses which the herd suffered about balanced its gain, and 

 there was neither increase nor diminution. From the nature of the losses which the 

 herd is subject, to we may infer that in its less crowded condition within the past 

 few years, they have been somewhat mitigated. This would be especially true of the 

 loss through the parasitic worm and through tights and struggles on the breeding 

 grounds. Under normal conditions in its present state, the herd might be expected 

 to increase by a slight margin each year. That it does not so increase is due to 

 the action of pelagic sealing. The measure of this possible increase in the herd is 

 the margin of ditlereuce between the number of 3 year-old females which enter the 

 breeding grounds in any season and the total number of deaths resulting to the adult 

 breeding herd from old age and the incidents of the sea. 



DEATH FROM OLD AGE. 



If we assume for the breeding female an average life of thirteen years, this 

 would give a breeding life of ten years, and the death rate from old age must each 

 year amount to about Id per cent of the breeding herd. On the other hand, it is 

 dear from the proportion between the breeding herd of 130,000 and the (juota of 

 20,000 for the present year that the proportion of pups which survive from any birth 

 rate to the age of 3 years is about one-third to one-fourth of the total number. The 

 quota of the present time is therefore roughly a measure of the gain of the herd, as 

 an approximately equal number of young females must survive. 



A HYPOTHETICAL CASE. 



As an illustration, let us assume for any given year a total breeding herd ot 

 180,000 cows. Of these 150,000 would be adults and 30,000 j'oung cows coming into 

 the herd for the first time as breeders and rei)reseuting the normal gain of the herd. 

 Assuming that, as a result of storms at sea, old age. and attacks of enemies, 10 per 

 cent of the herd are lost in the winter migration, this would mean the absence of 

 18,000 animals for the succeeding season, to cover which and provide for continued 

 increase the herd receives a gain of 30,000 young animals. The net gain to the herd 

 is. therefore, li.',()(Mi breeding females. This is a liberal estimate of gain. 



THE POSSIBLE ABSTRACTION OF FEMALES. 



If the killing of female seals produced only the direct loss entailed by their 

 absence, this removal of 12,000 females from the hyjiothetical stock of ISO 000 

 breeders would leave the herd in a state of equilibrium. But for each life thus lost 

 results the death of an unborn pup, and with such part of the 12,000 females as are 

 taken in Bering Sea nursing pups die also. This secondary loss is felt later in a 



