Introduction : Variation. 
77 
rations without number applies equally well here. The habits of Merop, are 
very different from those of Prionitimis ; the Parrot breeds m holes in tiees, but 
the Bee-eater forms a burrow, like the hole of a mouse or rat, for a o 
one to three metres in a bank of sand or earth. The friction caused by t e 
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-paper, a ragged simi i- 
tude of a Slerops-rectxix may be obtained. ') 
Other cases. If once the theory that the racket-tail-feathers of Pnomtiirus 
are the inherited results of attrition is admitted, a principle 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 Hmmdo, 
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 o 
the body forcing the webs apart at the tips of the featheis and so forming a 
little notch, just as is done by rubbing the tip of a feather on blotting paper 
or by knocking the tip gently with the finger; the stiff, tapering tails of Wood- 
peckers and Nasiterna, stimulated to strong growth and worn down to shape by 
the habit of using them as a prop in climbing; the curiously attenuated firat 
primaries of many Hornbills and Pigeons, so shaped by the friction caused in 
flight to these reduced quills ivhich 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 I he 
friction may have acted as a stimulus to the lengthening of these eat eis 
which are far larger and stronger than contour-feathers. Other parts may e 
modified in the same manner as feathers by the inherited effects o wear an 
tear: such as the bills of Anastonms, Esacus and Demicgretta, 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); sun 
of the head of the Cock, drawn out into a comb and with the formats e feather 
See also Meyer's remarks on and figures of the two lengthened middle tail-feathers of Paradisea 
mimr L. in Ahh. Ber. Mus. Dresden 1898/9 Nr. 2 p. 44 plate II. 
