AY13S, 
26 
at considerable ' lengtlM though concisely ^nongh when we re^ 
member that in Birdsmore than in any other animals are these 
characters diversified and conspicuous, and that there is scarcely 
an ornithologist of repute but has had something new to say of 
them. 
After briefly mentioning some sexual Differences in Structure 
which seem to depend on Differences in Habit, as the bills of 
Gr'ypus, Neomorphaj and even Carduelis, Mr. Darwin (p. 40) 
notices such differences as are connected with the extreme 
Pugnacity of the Males, citing among many other, instances 
those in Machetes * (the only case among Birds of a structure 
serving as a shield), the polygamous Gallince, some of the Art:- 
seres, and especially Palamedea. Then are considered the Sounds 
made by Birds, whether Vocal (p. 51) — of which most of the Pas- 
seres, some species of Tetrao, Cephalopterus, Gy gnus, and Grus fur- 
nish examples, — or Instrumental (p. 61), as illustrated by Tetrao 
again, Upupa, Scolopax gallinago, some Trochilidce, and Pipra 
deliciosa. Love-Antics and Dances, as performed especially by 
Chlamydera and its allies, form the next branch of the subject 
(p. 68), to which naturally follows the question of Decoration 
(p. 71), whether permanent or seasonal, the last leading logically 
to remarks on Moulting (p, 80), and a separation of those birds 
which have a Double Moult into five groups — those wherein 
(i.) the sexes are generally alike all the year round, (ii.) the 
sexes resemble each other though undergoing a slight change 
according to season, (iii.) the seasonal change is very great, 
(iv.) the males resemble the females for most part of the year, 
hut undergo a change during the rest, or (v.), the sexes being 
always unlike, the males undergo a greater change at each re- 
current season. Proof of the Display of their plumage by Males 
(p. 86) concludes this chapter. 
The next chapter begins with Choice as exercised by the Pemales 
(p. 99), and then shows (p. 100) that Courtship is often a prolonged 
affair, and leaves many Unpaired Birds (p. 103), Then the 
Mental Powers of Birds are considered (p. 108), and the attach- 
ment they feel for their comrades and owners ; their faculties of 
observation, especially as regards colours and ornaments, all 
receive due illustration. Numerous facts bearing on the Pre- 
ference of Females for Particular Males are next cited (p. 113) ; 
and then we come back (p. 124) to the old question of Variability 
and Inheritance as the foundation of Selection, whether Natural 
or Sexual ; and this gives rise (p. 132) to one of the most re- 
markable and ingenious ideas of the many which pervade the 
whole work, that which suggests the origin and formation of 
■i’ • ' 
* [Neither in this nor in his other works does Mr. Darwin seem to have 
noticed sufficiently the individual variability of the male of this bird, which 
in this respect is believed to stand alone among animals. — Ed.] 
