Chap. XVIII. EQUAL TRANSMISSION. 297 



or are excited by, the ornaments and colours displayed 

 before them. There is, however, a striking parallelism 

 between mammals and birds in all their secondarv sexual 

 characters, namely in their weapons for fighting with 

 rival males, in their ornamental appendages, and in their 

 colours. In both classes, when the male differs from the 

 female, the young of both sexes almost always resemble 

 each other, and in a large majority of cases resemble the 

 adult female. In both classes the male assumes the 

 characters proper to his sex shortly before the age for 

 reproduction ; if emasculated he either never acquires 

 such characters or subsequently loses them. In both 

 classes the change of colour is sometimes seasonal, and 

 the tints of the naked parts sometimes become more 

 vivid during the act of courtship. In both classes the 

 male is almost always more vividly or strongly coloured 

 than the female, and is ornamented with larger crests 

 either of hair or feathers, or other appendages. In a 

 few exceptional cases the female in both classes is 

 more highly ornamented than the male. With many 

 mammals, and at least in the case of one . bird, the 

 male is more odoriferous than the female. In both 

 classes the voice of the male is more powerful than that 

 of the female. Considering this parallelism there can be 

 little doubt that the same cause, whatever it may be, 

 has acted on mammals and birds ; and the result, as far 

 as ornamental characters are concerned, may safely be 

 attributed, as it appears to me, to the long-continued 

 preference of the individuals of one sex for certain in- 

 dividuals of the opposite sex, combined with their suc- 

 cess in leaving a larger number of offspring to inherit 

 their su23erior attractions. 



Equal transmission of oi^namental characters to both 

 sexes. — With many birds, ornaments, which analogy leads 



