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Show 62 DARWINISM CJJAI'. sufficient, which, however, is not often the case. The accompanying diagram exhibits the actu~l differences of size in five organs which occur in five spe~ICs taken al.most at random from this catalogue. Here, agam, we percmve that the variation is decidedly large, even among a very small number of specimens; while the facts all show that there is no ground whateve: fo.r .the comn~on assumption that ~atural species consist of md1~1duals whiCh ~~e n~arl! all, abke, or that the variations whiCh occur are mfimtes1mal or even "small." The proportionate Number of Individuals which pr·esent a considerable amount of Va1·iation. 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 diagrams is to show the number of individuals which vary considerably 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 Agelreus phceniceus, this approach to an equable spreading of the variations is still more apparent; while in Fig. 12, where fifty-eight specimens of Cardinalis trt VARIABILITY OF SPECIES IN A STATE OF NATURE 63 virginianus are registered, we see a remarkable spreading onL of the spots, showing in some of the characters a tendency to segregation into two or more groups of individuals, each varying considerably from the mean. In order fully to appreciate the teaching of these diagrams, • VARIATION OF IOTERUS BALTIMORE. 20.c3' rAil. • • • • • • • •• ••• •• •• •• • • • g . •• •• •••••• •• •• Tar us. • ••• •• • • ••• •••••• Midd e Toe . • • •• •• • ····l··· • Hind ~oe. • .· ••·J••·•· • Bit~~ Length. . n=r·=· Bill, ~idth. ••••••• • ••••• • 0 • F10. 9. • • • we must remember, that, whatever kind and amount of variations are exhibited by the few specimens here compared, would be greatly extended and brought into symmetrical form if large numbers-thousands or millions-were subjected to the same process of measurement and registration. We know, from the general law which governs variations from a mean value, that with increasing numbers the range |
