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 shoAv 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 

 occiu- 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 Avhich 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), shoA\'ing the variation 

 among forty males of Agelseus phreniceus, this approach to an 

 equable spreading of the variations is still more apparent ; 

 while in Fig. 12, where fifty- eight specimens of Cardinalis 



