BIOGRAPHICAL SKETCH lxix 
it will be seen that it is hopeless to try to advance in the opposite 
direction, from the bars forward to the checkered condition. No 
variations will appear in that direction, but such as do appear 
will take the opposite direction, tending to diminish the width of 
the bars and to weaken their color. It is in this way that we must 
account for the existence of some fancy breeds in which the bars 
have been wholly obliterated. The direction of evolution can 
never be reversed. 
“T have tried both experiments for eight years, and as both tell 
the same story as to the direction of variation, I am satisfied that 
further experiments will not essentially modify the results.” 
After tracing wing bars of diverse kinds to checkers, the origin 
of the checkers was traced from a still earlier and universal avian 
character. 
“Tt consists of a single dark spot occupying the centre of the 
exposed part of each feather. In the course of evolution, this 
spot has been divided into two lateral spots by the disappearance 
of pigment along the shaft, beginning at the apex of the feather 
and advancing gradually inward. The old Turtle-Dove charac- 
ter thus passes by a continuous process of division into the Rock 
Pigeon pattern, consisting of two checkers on each feather, more 
or less completely separated. The evidences showing such a 
gradual transmutation are still to be seen, and in such profusion 
as to wholly exclude doubt. Hundreds of species have been 
formed in this simple way, leaving no room for the claim of sud- 
den, nontransitional mutations. 
_ “The transitional stages between the Turtle-Dove pattern and 
the checkered pattern of the Rock pigeons, are exhibited not only 
as we pass from one species to another, but often as we advance 
from the juvenal to the adult plumage; and frequently they may 
be seen in different parts of one and the same individual plumage. 
“A still older character than the Turtle-Dove spot is seen in the 
cross-bars, or fundamental bars, that appear to mark all feathers of 
all species of birds. These bars were first noticed in pigeons in the 
summer of 1903, and were soon found to be common to allspecies 
of pigeons and birds in general. From these fundamental feather- 
bars or their secondary derivatives, a multitude of specific char- 
