1 82 PROBLEMS OF EVOLUTION 



considerable activity, and find their way from pond to pond and 

 stream to stream. Accordingly, among them we find the call- 

 note largely developed among the males. 



When the great evolution from the low reptile world took 

 place, the conversion of the fore-limb into a wing was the 

 greatest change. But among the further changes necessitated 

 by it was the development of means of recognition, whether 

 addressed to the eye or to the ear. It follows, as a matter of 

 course, that both eye and ear had to grow in keenness. In 

 deciding whether a particular feature is a recognition mark, or 

 whether it serves some other purpose, or whether it is merely 

 due to correlation with some useful structure, and itself serves 

 no purpose at all, we may make very great mistakes. When Dr 

 Wallace adopted the view that the white tails of rabbits were 

 " signal flags of danger," some people were struck by the ingen- 

 uity of the suggestion, others laughed at it as absurd. In any 

 case, we may regard it as proved that the males in many species 

 in widely separated classes of animals call to the females, and 

 though the case for recognition marks appealing to the eye is 

 not so strong, yet it is by no means weak. 



Dis- Myself, I attach great weight to the following argument. In 

 ^ r< ^ s t ^ iat are plain-coloured, with the frequent consequence that 



distinctive several species are very nearly alike, we find, as a rule, a charac- 

 often ter * st ' c son g developed in each species. On the other hand, when 



alternative the plumage of closely allied species affords an easy distinction, 

 the songs and call-notes are often very difficult to distinguish. 

 The Warblers are a very plain race, and among them the power 

 of song is brought to great perfection. Two well-known Warblers, 

 the ChiffchafF (Phylloscopus rufus) and the Willow-wren (P. trochy- 

 lus) are very much alike, so that, without a very close inspection, 

 it is difficult to tell them apart. But directly the song of the two 

 is heard, the doubt is removed. The Wood-wren (P. sibilatrix), 

 the Reed-warbler (Acrocephalus streperus), the Marsh-warbler (A. 

 palustris), the Sedge-warbler {A. phragmitis\ the Grasshopper- 

 warbler (Locustella tuevia), the Blackcap (Sylvia atricapit/a), the 

 Garden-warbler (S. hortensis), the Whitethroat (5. cinerea), the 

 Nightingale (Daulias luscinia}, may all be put down as Warblers : 



