VOL. u.] Colors of West Coast Mammals. ; 207 
their environment would escape destruction, while those in whom the 
lighter colors preponderated would naturally be eliminated. More- 
over these darker females would naturally pair with the darker 
males, there being many facts to indicate. that, other things being 
equal, like mates with like. The paler males also would be elim- 
inated so that there would be more dark males to choose from. 
It is at any rate not unreasonable to suppose that these darker 
females would pair with mates at least not paler than the average 
shade of the male. This would give us a generation in which the 
proportions would be something like the following: 
MALES. FEMALES. 
Es oe oy eee shade Pesach ees 10 15 
ss ee ee ree 60 50 
CRS EST ES SRNR tl a et un a Pa 30 35 
In other words, the tendency of natural selection would be, if no 
ae other factor were at work, to make the female approach the color 
of the male, for the females which did come the nearest to the shade 
of the male would be the best protected and consequently survive. 
But the fact must not be lost sight of that the katabolism of the 
‘male in this species induces it to expend its surplus energy in an 
excess of pigment over that of the female. There would thus be 
a tendency to maintain the average proportion between the two 
sexes. Therefore, other things being equal, the average color of 
_ the males of the third generation would be shown as follows: 
IST GENERATION. MALE OF FEMALE OF 
MALE. FEMALE. 3D GENERATION. 2D GENERATION. 
BROWN 55 ei ise TOOK 20 < x - 15 
PROS lass Big 04 Gor 5215 40 i x eee 50 
x WHE ccsicd isos 30) 4 40 zs x : 35 
In which case the average male of the third generation would have 
colors in the proportion of brown 7.5, black 75, and white 26.25, 
_ which shows a great increase in the proportion of black. Of course, 
owing to the great number and complexity of the limiting and modi- 
_ fying conditions in a state of nature no such rapid change in color 
_ would be possible, but on the contrary the change would be in- 
finitely gradual. All I have attempted to show is that when the 
_ male takes the lead in a variation, if it be a modification which is 
ae of equal use to both sexes, there will be a tendency for the female to 
follow the male, but, in following its mate, owing to the difference 
in n metabolism between the two sexes, to keep pushing it in advance 
