OTHER THEORIES Of SEXUAL COLOURINO 331 



Certain general considerations which support Darwin's 

 theory of Sexual Selection 



There are also one or two general facts which seem 

 to me to strongly support the theory of Sexual Selec- 

 tion, and to oppose any theory which is not based on 

 selective breeding. 



Sexnal Colours only developed in species which court 

 by day or twilight, or have probably done so at no 

 distant date 



The appearance of beautiful colours and patterns, 

 which are displayed in courtship, invariably occurs in 

 diurnal or partially diurnal animals. The colours 

 only appear when the conditions for female preference 

 are present also. If we compare butterflies with moths, 

 or moths which fly by day and twilight with those 

 which fly in darkness, we find that brilliant tints and 

 ornamental patterns are only found when there is light 

 enough for the female to see them. The consideration 

 of apparent exceptions will be found to support the 

 argument. The same evidence may be drawn from 

 birds and other animals. If, however, such colours 

 were merely the symptom of vitality, we should not 

 expect to find this invariable relationship between the 

 colours of one sex and the conditions for seeing it in 

 the other. 



