550 THE WONDER OF LIFE 



reflect the light like mirrors and are effective guide-marks. 



Attractive Coloration. A fifth use of coloration 

 is to add to the ensemble of attractiveness which one sex 

 has for the other, and which, by stimulating sexual interest 

 and increasing sexual excitement, makes pairing more effec- 

 tive. It is usually the male who is the more decorative and 

 brilliant, but there are exceptional cases of the reverse. 

 The tail of the peacock is a masterpiece in this kind of 

 coloration, and the decorativeness of male Birds of Para- 

 dise and Lyre-Birds and Pheasants is hardly less transcen- 

 dent. The habits of Bower-birds and some other birds 

 which collect brightly-coloured or shining objects, suggest 

 that an appreciation of colour, as colour, is not wanting, 

 but in thinking of courtship coloration it is probably safer 

 not to be too analytic, crediting the female bird with too 

 much in the way of particulate aesthetic discernment. It is 

 probably the total impression of agility, beauty, vigour and 

 other qualities which kindles or fans the fire of sexual 

 excitement in the coy female. 



The brilliant coloration of the males is in some measure 

 latent in the females, as is shown in cases where an old 

 female bird, or one with an abnormal ovary, begins to put 

 on a masculine dress. The masculine characteristics are, 

 as it were, seeds which will not normally develop except 

 in male soil. They are parts of the inheritance, but they 

 do not start developing except in appropriate soil and in 

 response to appropriate stimulus. It has been shown 

 experimentally that the stimulus, in some cases at least, 

 is furnished by the ' hormones ' or internal secretions of the 

 reproductive organs which are diffused by the blood through- 

 out the body when the organism becomes adolescent or 

 when the breeding season sets in. It is interesting to find 



