Introduction: Variation. 
75 
yielding pliancy of the tip making it difficult to remove the web from this 
point without cutting off the end of the feather altogether. In Nature any feather 
of sufficient stiffness, prolonged so as to stand out beyond the other feathers, will 
be liable to such a process as this, attrition against the twigs of trees, the walls 
of their nesting holes etc. , supplying the place of the knife. Assumed that the 
two middle tail-feathers of Prioniturus were originally a little longer than the 
rest^), the ends, if sufficiently prolonged, are liable to attrition; and a narrowing 
of the tips, such as is now seen in the young birds (pi. V, fig. 1 ) , will result. 
The friction at the ends of the feathers causes irritation to the roots; an in- 
creased supply of blood ensues there, with the result of an increased size of these 
feathers. These longer feathers are more liable to attrition, and half- formed 
rackets (pi. V, fig. 3) take shape ; the increased irritation and consequent 
lengthening of the feather results in the production of other stages (pi. V, fig. 4), 
up to the most advanced development of the present time (pi. VI, fig. 1). Yet 
the striking features shown in the plates were not obtained in one generation, 
as has been proved; on the other hand this appears to have been a process 
of ages, more and more advanced results being obtained in successive 
generations and transmitted by heredity. The simplest stages of this for- 
mation are displayed by young birds in first plumage which in respect of the 
tail probably resemble the first ancestors of the genus (pi. V, fig. 1); the second 
moult, when the webs are often quite absent on the shafts of the rackets, which 
are about half the full length in old birds, seems to show a later period in the 
history of the race ; while the highest development of th'ese feathers, as seen in 
old birds (especially males) of the present day, is probably the most recent stage 
in the evolution of the genus. 
The following are the arguments in proof that these rackets are the inherited 
efi'ects of attrition: 
1. It has been shown that such can easily be formed artificially by scraping, 
the size of the spatule depending upon the stiffness of the feather. 
2. Where the shafts are not exposed to attrition they are not bare. 
It is only on the projecting part of the middle tail-feathers that the shafts are 
bare; and as far as the ends of the lateral feathers, by which the middle ones 
are protected from attrition, they are fully webbed. If the bareness were due 
to something else, it might be expected that the naked shaft would not in 
every species^) arise just at this point of the tail, but sometimes much higher 
up, or sometimes much lower down®). 
3. Rackets do not occur on unexposed feathers sheltered from attrition. 
1) A very common condition in birds. 
2) We have examined P. lueonensis, cyaneiceps, suluensis, diseums, flavicans, platurus, and the plate of 
verticalis. 
3) Genera in which the racket-feathers are longer and consequently heavier (e. g. Bhringa) usually have 
larger spatules and the attenuation of the webs on the shaft continued towards the base of the tail — a result 
of friction upon the other feathers. 
10 * 
