The causes of death here enumerated are natural and inherent in the conditions under which the herd exists. That some of them were not known or fully understood until the investigations of 1896 and 1897 does not make them new or recent in their action. They have been constant factors, acting with greater intensity in the past when the herd was larger and more crowded upon its breeding grounds. Photographs taken in 1891 and 1892 show that the parasitic worm was then doing its deadly work, and more extensively in proportion as the herd was larger. For 1,495 pups dead from this cause counted by us on Tolstoi sand flat in 1896, 4,000 were counted on the same ground by the British commissioner of 1892. Moreover, the bones of innumerable pups on ground already abandoned in that year by the declining herd attest the existence of this cause of death prior to that time. We have no reason to suppose that it has not always preyed upon the herd. Death by trampling must at present be at a minimum on account of the scattered condition of the rookeries. The storms of winter and pelagic enemies must, of course, take toll in proportion to the number of animals.
But the significant fact shown by this proposition is that the gain of the herd must be small at best under such a natural death rate. We may suppose these natural losses to have been the checks which in a state of nature prevented the indefinite increase of the herd, When, therefore, to this total loss of from two thirds to four fifths of the entire birth rate before breeding age is attained, we add the tremendous artificial loss through the destruction of gravid and nursing females resulting from pelagic sealing, it is not to be wondered at that the equilibrium was broken and the herd sent on a rapid decline.
These figures are based upon counts of all the breeding families on both islands for each season. On certain rookeries the live and dead pups were counted. In this way an average size of family was obtained which was used to complete the census where pups could not be counted.