62 
DARWINISM 
CHAP. 
sufficient, which, however, is not often the case. The 
accompanying diagram exhibits the actual differences of size 
in five organs which occur in five species taken almost at 
random from this catalogue. Here, again, we perceive that 
the variation is decidedly large, even among a very small 
number of specimens; while the facts all show that there is 
no ground whatever for the common assumption that natural 
species consist of individuals which are nearly all alike, or 
that the variations which occur are “ infinitesimal ” or even 
“ small.” 
The proportionate Number of Individuals which present a 
considerable amount of Variation. 
The notion that variation is a comparatively exceptional 
phenomenon, and that in any case considerable variations 
occur very rarely in proportion to the number of individuals 
which do not vary, is so deeply rooted that it is necessary to 
show by every possible method of illustration how completely 
opposed it is to the facts of nature. I have therefore 
prepared some diagrams in which each of the individual birds 
measured is represented by a spot, placed at a proportionate 
distance, right and left, from the median line accordingly as 
it varies in excess or defect of the mean length as regards the 
particular part compared. As the object in this set of dia¬ 
grams is to show the number of individuals which vary con¬ 
siderably in proportion to those which vary little or not at 
all, the scale has been enlarged in order to allow room for 
placing the spots without overlapping each other. 
In the diagram opposite twenty males of Icterus Baltimore 
are registered, so as to exhibit to the eye the proportionate 
number of specimens which vary, to a greater or less amount, 
in the length of the tail, wing, tarsus, middle toe, hind toe, and 
bill. It will be noticed that there is usually no very great 
accumulation of dots about the median line which shows the 
average dimensions, but that a considerable number are spread 
at varying distances on each side of it. 
In the next diagram (Fig. 10), showing the variation 
among forty males of Ageloeus phoeniceus, this approach to an 
equable spreading of the variations is still more apparent; 
while in Fig. 12, where fifty-eight specimens of Cardinalis 
