52 
DARWINISM 
Chap. 
song sparrow (Melospiza melodia), the fox-colourecl sparrow 
(Passerella iliaca), the swamp sparrow (Melospiza palustris), the 
black and white creeper (Mniotilta varia), the water-wagtail 
(Seiurus novseboracencis), in Turdus fuscescens and its allies, the 
difference in the size of the streaks is often very considerable. 
In the song sparrow they vary to such an extent that in some 
cases they are reduced to narrow lines; in others so enlarged 
as to cover the greater part of the breast and sides of the body, 
sometimes uniting on the middle of the breast into a nearly 
continuous patch.” 
Mr. Allen then goes on to particularise several species in 
which such variations occur, giving cases in which two speci¬ 
mens taken at the same place on the same day exhibited the 
two extremes of coloration. Another set of variations is 
thus described : “ The white markings so common on the wings 
and tails of birds, as the bars formed by the white tips of the 
greater wing-coverts, the white patch occasionally present at 
the base of the primary quills, or the white band crossing 
them, and the white patch near the end of the outer tail- 
feathers are also extremely liable to variation in respect to 
their extent and the number of feathers to which, in the same 
species, these markings extend.” It is to be especially noted 
that all these varieties are distinct from those which depend 
on season, on age, or on sex, and that they are such as have 
in many other species been considered to be of specific 
value. 
These variations of colour could not be presented to the eye 
without a series of carefully engraved plates, but in order to 
bring Mr. Allen’s measurements , illustrating variations of size and 
proportion, more clearly before the reader, I have prepared a 
series of diagrams illustrating the more important facts and 
their bearings on the Darwinian theory. 
The first of these is intended, mainly, to show the actual 
amount of the variation, as it gives the true length of the 
wing and tail in the extreme cases among thirty specimens of 
each of three species. The shaded portion shows the minimum 
length, the unshaded portion the additional length in the 
maximum. The point to be specially noted here is, that in 
each of these common species there is about the same amount 
of variation, and that it is so great as to be obvious at a glance. 
