Introduction: Variation. 77 



rations without number applies equally well here. The habits of Merops are 

 very different from those of Prioniturus ; the Parrot breeds in holes in trees, but 

 the Bee-eater forms a burrow, like the hole of a mouse or rat, for a depth of 

 one to three metres in a bank of sand or earth. The friction caused by the 

 sand, against which the terminal portion of the feather is chiefly brushed, seems 

 sufficient to account for this peculiar shape. If a feather of ordinary shape be 

 taken, and rubbed and drawn between two sheets of sand-])aper, a ragged simili- 

 tude of a Mero/w-rectrix may be obtained, 'j 



Other cases. If once the theory that the racket-tail-feathers of Priomtumn 

 are the inherited results of attrition is admitted, a ])rinciple is arrived at by 

 which a host of other cases are capable of explanation. Among feather-forma- 

 tions may be mentioned: the bifid tips of the remiges of Merops and Hirundo, 

 explicable by the habit of these birds of supporting themselves on their wings 

 when commencing their nests (for Merops, see p. 252, note , the oscillation of 

 the body forcing the webs apart at the tips of the feathers and so forming a 

 little notch, just as is done by rubbing the tip of a feather on blotting-pa])er 

 or by knocking the tip gently with the finger ; the stiff, tapering tails of Wood- 

 peckers and Nasiternu, stimulated to strong gi-owth and worn down to shape by 

 the habit of using them as a pro]) in climbing; the curiously attenuated first 

 primaries of many Hornbills and Pigeons, so shaped by the friction caused in 

 flight to these reduced quills which lie under the other remiges, against which 

 they vibrate and by which they are rubbed; the narrowing of the outer 

 webs of the lateral tail-feathers of all birds and the gradual increase in 

 width of these webs from one feather to another until on the two middle feathers 

 they are of approximately equal width, the middle feathers being protected by 

 the lateral ones from the friction of objects against which the tail is repeatedly 

 getting brushed, the lateral feathers being exposed to this attrition, — most of all 

 the outermost pair in which the outer web is narrowest. Also in the narrowing 

 of the outer webs of the remiges, though feathers of this shape are apparently 

 essential to flight, mechanical attrition, caused by the rush of air in flight, may 

 have worked together with natural selection in determining their shape. The 

 friction may ha\e acted as a stimulus to the lengthening of these feathers 

 which are far larger and sti'onger than contour-feathers. Other parts may be 

 modified in the same manner as feathers by the inherited effects of wear and 

 tear: such as the bills of Anastomus, Esacus and Demiegretta , worn away so as 

 no more to close properly by the rough shells and crustaceans upon which the 

 birds feed; the bill of adult Hornbills not meeting for a space where the bird 

 lays hold of objects in climbing and feeding and even swings from them on 

 occasion suspended by its bill see, Legge, B. Ceylon 1880, p. 274}; the skin 

 of the head of the Cock, drawn out into a comb and with the formative feather- 



'j See also lleyer's remarks on and figures of the two lengthened middle tail-feathers of Paraditea 

 minor etc. in Abh, Ber. Mus. Dresden 16'JS/9 Xr. 2 p. 44 plate IL 



