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SKXUAL DIMORPHISM 



are practised earlier. We know that in domestic poultry 

 young cocks begin to fight long before they are sexually 

 mature. Humming-birds very possibly reach maturity at a 

 very early age, and possibly begin to practise their special 

 movements as soon as they can fly. In this case the develop- 

 ment of their special plumage would likewise be hastened. 

 In another species it is mentioned by Darwin that both male 

 and female are brilliantly but very differently coloured, and 

 the young begin to show the difference from the first. This 

 case I cannot pretend to explain satisfactorily. As I have 

 remarked in reference to other cases, we know less of the 

 influences that affect colour than of those which affect size of 

 organs, although there are indications that greater sexual 

 activity is one of the conditions influencing the development 

 of pigment. I have no doubt, however, that in all cases 

 where inheritance of certain characters is limited to one sex, 

 it is because those characters are due to conditions of life 

 which affect that sex, and not the other. 



I have already referred to Mr. Belt's interesting 

 description of the very definite erection and display of 

 special plumage in male humming-birds, namely, Florisuga 

 mellivora. The description is quoted in Darwin's Descent of 

 Man and Sexual Selection. Two males were displaying their 

 ornaments before a female sitting on a branch. One after 

 the other shot up high into the air and slowly descended with 

 expanded tail. The tail was white, and when expanded 

 covered more space than the rest of the bird; it was 

 " evidently the grand feature of the performance." 



The special modifications of the male in the Night-jars, 

 Caprimidgidw, consist in the elongation of particular feathers, 

 different feathers being enlarged in different species. The 

 important facts about these characters are scattered separately 

 in Darwin's work, but when brought together they suggest 

 an interpretation different to that which he maintained. 



