x COLOURS AND ORNAMENTS CHARACTERISTIC OF SEX 275 
partners ; and Dr. Wallace of Colchester, who has largely bred 
the fine Bomhyx cynthia, confirmed this statement. Among 
butterflies, several males often pursue one female, and Mr. 
Darwin says, that, unless the female exerts a choice the 
pairing must be left to chance. But, surely, it may be the 
most vigorous or most persevering male that is chosen, not 
necessarily one more brightly or differently coloured, and 
this will be true “ natural selection.” Butterflies have been 
noticed to prefer some coloured flowers to others; but that 
does not prove, or even render probable, any preference for 
the colour itself, but only for flowers of certain colours, on 
account of the more agreeable or more abundant nectar 
obtained from them. Dr. Schulte called Mr. Darwin’s atten¬ 
tion to the fact, that in the Diadema bolina the brilliant blue 
colour surrounding the white spots is only visible when we 
look towards the insect’s head, and this is true of many of 
the iridescent colours of butterflies, and probably depends 
upon the direction of the striae on the scales. It is suggested, 
however, that this display of colour will be seen by the 
female as the male is approaching her, and that it has been 
developed by sexual selection . 1 But in the majority of cases 
the males follow the female, hovering over her in a position 
which would render it almost impossible for her to see the 
particular colours or patterns on his upper surface ; to do so 
the female should mount higher than the male, and fly 
towards him—being the seeker instead of the sought, and this 
is quite opposed to the actual facts. I cannot, therefore, 
think that this suggestion adds anything whatever to the 
evidence for sexual selection of colour by female butterflies. 
This question will, however, be again touched upon after we 
have considered the phenomena of sexual colour among the 
vertebrata. 
Sexual Coloration of Birds. 
The general rule among vertebrates, as regards colour, is, 
for the two sexes to be alike. This prevails, with only a few 
exceptions, in fishes, reptiles, and mammalia; but in birds 
diversity of sexual colouring is exceedingly frequent, and is, 
not improbably, present in a greater or less degree in more 
1 Darwin in Nature, 18S0, p. 237. 
