128 PECK. (VoL. III. 
In most cases the dove-cote pigeon possesses six free caudal 
vertebrz and the coccyx; but not infrequently specimens are 
found with seven free vertebrz, the fan-tail with eight or seven, 
while sometimes cases are found with as few as five. In a paper 
on the gallinaceous birds,! W. K. Parker says: ‘I do not set so 
much value on the number of caudal vertebre, as the last isa 
series, and the tail is very apt to vary in the number of those 
which shall be swallowed up in the terminal piece. The sacral 
are easily distinguished from the caudal in these birds, as even in 
the pigeons the caudal are no longer pneumatic; that is a better 
character even than the coalescence of a vertebra with those 
that precede it.” Thus in the pigeons the variability extends 
not only to members of different orders of birds, but is marked 
in the same species and may be more or less fixed by selection, 
as in the fan-tails, which ordinarily have eight or nine caudal 
vertebrz, according to Darwin,? and possibly ten. The coccyx 
from the very nature of its composition is of varying length and 
curvature ; these features being determined perhaps by the more 
or less complete fusion of a vertebra to its anterior end, while 
the connective tissue which attaches to it and bridges over from 
it to the adjoining vertebrz, forms a convenient medium of ossi- 
‘fication tending toward the obliteration of the last and smallest 
vertebra in the compact mass of connective tissue for the inser- 
tion of the rectrices. But it can hardly be said that the num- 
ber of free vertebrze depends upon the freedom or fusion of the 
last one with the coccyx, for in this case we should expect to 
find a long coccyx following a short series of caudal vertebre, 
and a short coccyx where the most vertebrz are free from it; 
whereas the facts would indicate that a large number of vertebrz 
is followed by a longer coccyx, so that in the cases having seven 
or eight free vertebrz the coccyx is longer than in those possess- 
ing five or six (cf. cin Figs 1-4), suggesting some other cause 
for the lengthening of the whole region, as a result of which 
more vertebrz are left detached from the terminal piece; so 
that as the whole region is lengthened or shortened the coccyx 
lengthens or shortens in about the same ratio. But it is often 
quite difficult to determine the exact length of the coccyx, on 
account of its imperfect union with the preceding vertebra. 
1 Transactions of the Zovlogical Society of London, Vol. V., p. 198. 
2 Animals and Plants under Domestication, Vol. I., p. 205. 
