Vol. XVII 
1900 
ALLEN, ‘ Aptosochromatism, 32 yi 
the black feathers of the breast which are untouched. The black 
and naked barbs shown in Fig. 2 give that portion of the breast 
its hairy or bristly appearance. 
An effort to assign a cause for this peculiar habit of the Sage 
Cock would be entering the purely speculative field, but the slid- 
ing of the widely distended air sacks over an uneven surface, 
together with the additional rumbling produced by the stiffened 
worn-off feathers, undoubtedly produce, to the ears of the bird at 
least, a volume of sound that is simply tremendous. A spectator, 
however, can hear nothing until the air sacks are collapsed. 
‘APTOSOCHROMATISM.’ 
BYs i A. ALLEN, 
In ‘ Science ’ of Feb. 23, rg00 (N.S., XI, pp. 292-299) Mr. F. J. 
Birtwell describes at considerable length, what he considers a case 
of ‘The Occurrence of Aptosochromatism in Passerina cyanea.’ 
He says: “ The following remarks upon Aptosochromatism of 
Passerina cyanea, although of insufficient importance to establish 
the phenomenon of color change without moult as a constant 
occurrence in the species, are conclusive enough, I am convinced, 
to prove the possibility of such a change, and are merely offered 
as such for what they may be worth.” Several paragraphs, by 
way of introduction, relate to the general subject, in which Mr. 
Birtwell regrets that “Individual error and dogmatism have greatly 
retarded honest effort in this most important branch of ornitho- 
logical science. It is a singular fact that certain individuals have 
conceived the idea that a feather once having passed its prema- 
ture condition is utterly disconnected with the vital system of the 
bird, and such individuals cling to this belief with a tenacity won- 
derful to behold.” His remarks, he tells us, “ are based chiefly 
upon observations conducted during the fall, winter, and early 
spring of 1898-99, upona captive bird.” He presents a table 
