THE PLAY OF ANIMALS. 233 



of the male and the female's answering cry. " These are 

 evidently a valuable addition to the means of recogni- 

 tion of the two sexes, and are a further indication that 

 the pairing season has arrived; and the production, in- 

 tensification, and differentiation of these sounds and 

 odours are clearly within the power of natural selection. 

 The same remark will apply to the peculiar calls of 

 birds, and even to the singing of the males. These 

 may well have originated merely as a means of recogni- 

 tion between the two sexes of a species and as an invi- 

 tation from the male to the female bird. When the 

 individuals of a species are widely scattered, such a call 

 must be of great importance in enabling pairing to take 

 place as early as possible, and thus the clearness, loud- 

 ness, and individuality of the song becomes a useful 

 character, and therefore the subject of natural selec- 

 tion." * Thus sexual selection would be absorbed in 

 natural selection, and Wallace advances two principles to 

 assist in the absorption. Many characteristic markings 

 and decorative colourings are, according to A. Tylor, 

 closely connected with anatomical structure. Since the 

 clearest colours show where the most important nerves 

 run, their intersections form all sorts of figures. And 

 " as the nerves everywhere follow the muscles, and these 

 are attached to the various bones, we see how it happens 

 that the tracts in which distinct developments of colour 

 appear should so often be marked out by the chief di- 

 visions of the bony structure in vertebrates, and by the 

 segments in the annulosa."f 



* Darwinism, p. 284. We see here how Wallace came to change 

 his mind about instinct. 



f Ibid., p. 290. Tylor, for instance, finds in the zebra's stripes 

 a picture of the spine and ribs. But why, then, is the symmetry 

 so soon lost in domestication f 



