1 74 Evolution and Adaptation 



is probably attempting to show is that the material for the 

 further action of sexual selection is already given ; but the 

 question may well be asked, if the external conditions have 

 done so much, why may they not have gone farther and pro- 

 duced the entire result ? 



Darwin makes the following suggestion to account for those 

 cases in which the female is the more highly colored : — 



" A few exceptional cases occur in various classes of 

 animals, in which the females instead of the males have 

 acquired well-pronounced secondary sexual characters, such 

 as brighter colors, greater size, strength, or pugnacity. 

 With birds there has sometimes been a complete transposi- 

 tion of the ordinary characters proper to each sex ; the 

 females having become the more eager in courtship, the 

 males remaining comparatively passive, but apparently select- 

 ing the more attractive females, as we may infer from the 

 results. Certain hen birds have thus been rendered more 

 highly colored or otherwise ornamented, as well as more 

 powerful and pugnacious than the cocks ; these characters 

 being transmitted to the female offspring alone." 



Then follows immediately the discussion as to whether a 

 double process of sexual selection may not be supposed to go 

 on at the same time. " It may be suggested that in some 

 cases a double process of selection has been carried on ; that 

 the males have selected the more attractive females, and the 

 latter the more attractive males. This process, however, 

 though it might lead to the modification of both sexes, would 

 not make the one sex different from the other, unless indeed 

 their tastes for the beautiful differed ; but this is a supposition 

 too improbable to be worth considering in the case of any 

 animal, excepting man. There are, however, many animals 

 in which the sexes resemble each other, both being furnished 

 with the same ornaments, which analogy would lead us to 

 attribute to the agency of sexual selection. In such cases 

 it may be suggested with more plausibility, that there has 



