Darwin's Theory of Sexual Selection 173 
body is higher in the male than in the female, accompanied 
in the case of man by a slower pulse. On the whole, the 
expenditure of matter and force by the two sexes is probably 
nearly equal, though effected in very different ways and at 
different rates,”’ 
Again: “From the causes just specified, the two sexes 
can hardly fail to differ somewhat in constitution, at least 
during the breeding season; and although they may be 
subjected to exactly the same conditions, they will tend to 
vary in a different manner. If such variations are of no 
service to either sex, they will not be accumulated and in- 
creased by sexual or natural selection. Nevertheless, they 
may become permanent if the exciting cause acts perma- 
nently ; and in accordance with a frequent form of inheritance 
they may be transmitted to that sex alone in which they 
first appeared. In this case, the two sexes will come to 
present permanent, yet unimportant, differences of character. 
For instance, Mr. Allen shows that with a large number of 
birds inhabiting the northern and southern United States, 
the specimens from the south are darker-colored than those 
from the north; and this seems to be the direct result of 
the difference in temperature, light, etc., between the two 
regions. Now, in some few cases, the two sexes of the same 
species appear to have been differently affected; in the 
Agcleus pheniceus the males have had their colors greatly 
intensified in the south; whereas with Cardzinalis virginianus 
it is the females which have been thus affected : with Quzs- 
calus major the females have been rendered extremely vari- 
able in tint, whilst the males remain nearly uniform.” 
The admissions contained in this statement would seem 
to jeopardize the entire question, for, if it is admitted that, on 
account of the difference in the constitution of the two sexes, 
the influence of the surrounding conditions would produce a 
different effect on them, it would seem that there is no need 
whatsoever for the theory of sexual selection. What Darwin 
