106 The American Naturalist. [February, 
Callorhinus ursinus (Northern Fur Seal). 
(a) Extreme sexual disparity. The males weigh three times 
as much as the females. (4) Ultra polygamous, the males main- 
taining a large harem, and guarding the females with the great- 
est vigilance and courage. In fact, this animal is the most 
polygamous of all the Mammalia. (c) Males fight with greatest 
desperation and persistence for females. 
A consideration of the above series will disclose the fact that 
there is a close and constant relation between polygamy and 
disparity in size among the Pinnipedia. It also indicates that 
this relation is a direct one, the disparity increasing part passu 
with the polygamy throughout the series. Another fact is ren- 
dered evident by this series, and that is that the combativeness of 
the males increases pari passu with sexual disparity and polygamy. 
These facts having been reasonably well established, it is pos- 
sible to construct a hypothetical history of events which will 
illustrate the successive stages by which a species might pass 
from a simply gregarious habit, in which monogamy, or at least 
promiscuity, prevails, to the extreme of polygamy practiced by 
the northern fur seal. Such a transition may be conceived to — 
take place by the following steps or gradations: 
Ist, An eminently gregarious species would offer more favor- 
able conditions for the introduction of polygamy than a non- 
gregarious species. Our point of departure in this part of the 
discussion would then be a gregarious, monogamous species. It 
the principles deduced from an examination of the series presented 
in the first part of this paper be correct, this species should also 
be one in which.there is little sexual disparity, and little or no 
fighting among the males for the possession of the females. All 
of the above conditions seem to be fulfilled in the case of the 
walrus (Odobenus rosmarus). This species will then stand for our 
point of departure. : 
2nd, The gregarious habit of the walrus offers a constant 
opportunity for a departure from the path of monogamous recti- 
tude. This fact is well illustrated in human affairs by the great 
6 Rikkase y 4. 4 a RA i 3 fight fifty or sixty battles during a single 
season., 
