254 THE DESCENT OF MAN. 



a somewhat different manner. The female has to expend 

 much organic matter in the formation of her ova, whereas 

 the male expends much force in fierce contests with his 

 rivals, in wandering about in search of the female, in exert- 

 ing his voice, pouring out odoriferous secretions, etc. ; and 

 this expenditure is generally concentrated within a short 

 period. The great vigor of the male during the season of 

 love seems often to intensify his colors independently of 

 any marked difference from the female.* In mankind, 

 and even as low down in the organic scale as in the Lepi- 

 doptera, the temperature of the body is higher in the male 

 than in the female, accompanied in the case of man by a 

 slower pulse, f 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. 



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 increased by 

 sexual or natural selection. Nevertheless, they may be- 

 come permanent if the exciting cause acts permanently; 

 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 char- 

 acter. 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 



*Prof. Mantegazza is inclined to believe ("Lettera a Carlo Dar- 

 win," "Archivio per FAnthropologia," 1871, p. 306) that the bright 

 colors, common in so many male animals, are due to the presence and 

 retention by them of the spermatic fluid; but this can hardly be the 

 case; for many male birds, for instance young pheasants, 'become 

 brightly colored in the autumn of their first year. 



f For mankind, see Dr. J. Stockton Hough, whose conclusions are 

 given in the " Pop. Science Review," 1874, p. 97. See Girard's 

 observations on the Lepidoptera, as given in the "Zoological Record," 

 1869, p. 347. 



