VARIATION 307 



conflict with its environment. Herein we may find a solution 

 of the problem of the existence of " incipient characters," so 

 commonly urged as an objection to the theory of natural selection. 



It is possible that a change of habit and habitat, even 

 though this be slight, may be the stimulating factor in produc- 

 ing what we call "species". These units exist, for us, only 

 when labelled, so to speak, by more or less tangible differenti- 

 ating characters, and, so far as birds are concerned, superficial 

 characters. 



Thus the Chiff-chaff and Willow-warbler, the Reed and the 

 Marsh-warbler, to select examples from among our native birds, 

 are to be distinguished one from another only by experts. But 

 they can be readily distinguished by means of their nests. 



It is possible, it is worth considering, whether such changes 

 in habit and habitat may not be at least one factor in causing 

 germinal disturbance which sooner or later expresses itself in 

 new combinations and permutations of colour areas, such as have 

 been described in the Swallows, and which give rise to new 

 "specific " characters. Later, and as a natural sequence of this 

 change of habit and habitat, deep-seated structural changes 

 take place, and new groups of species are thereby formed. 



A peculiarly suggestive light on the evolution of species is 

 furnished by the Great Titmouse {Parus major) and its near 

 ally the Grey Titmouse {Parus cinereus). With the general 

 appearance of the first-named all my readers must be familiar, 

 for it is a common British bird ; suffice it to say that its 

 coloration is a combination of blue, black, white and yellow. 

 The Grey Tit, on the other hand, though precisely similar in. 

 the pattern' o( the coloration, differs in the hue thereof, pear> 

 grey taking the place of blue, and white of yellow. The 

 fledgeling of the Great Tit, it is next to be noticed, differs from: 

 the adults — both sexes being coloured alike — in being duller ini 

 hue as to the blue and yellow, while the white of the face is 

 strongly tinged with yellow, and the black lacks the metallic 

 lustre, and is less in area, being absent on the side of the neck 

 and on the breast. The fledgeling Grey Tit is practically in- 

 distinguishable. Thus it would seem that the Grey Tit has 

 been derived from the Great Tit, the young repeating the 

 ancestral plumage of the latter, albeit imperfectly. 



In the Genera Oxynotus and Edoliiosoma — " Cuckoo-shrikes " 



