42 COLOUR. [iNTROD. 



shewn the smallest length of forceps was 25 mm., and the greatest 

 9 mm., the greatest frequency being grouped about 3'5 mm. and 

 7 mm. respectively. The mean form having forceps of moderate 

 length is comparatively rare. The size of the forceps of the 

 females scarcely varies at all, probably less than 1 mm. in the 

 whole sample. 



The number of cases is enough to fairly justify the acceptance 

 of these statistics and it is not likely that a greater number of 

 cases would much alter the shape of the curve. Here, therefore, 

 is a group of individuals living in close communion with each 

 other, high and low, under the same stones. No external circum- 

 stance can be seen to divide them, yet they are found to consist 

 of two well-marked groups. 



Before leaving these examples special attention should be di- 

 rected to the fact that the existence of a complete series of indivi- 

 duals, having every shade of development between the "lowest" and 

 the " highest " male, does not in any way touch the fact that the 

 Variation may be Discontinuous ; for we are concerned not with 

 the question whether or no all intermediate gradations are possible 

 or have ever existed, but with the wholly different question 

 whether or no the normal form has passed through each of these 

 intermediate conditions. To employ the metaphor which Galton 

 has used so well — and which may prove hereafter to be more than 

 a metaphor — we are concerned with the question of the positions 

 of Organic Stability; and in so far as the intermediate forms are 

 not or have not been positions of Organic Stability, in so far is 

 the Variation discontinuous. Supposing, then, that the "high" and 

 " low " males should become segregated into two species — a highly 

 improbable contingency — these two species would have arisen by 

 Variation which is continuous or discontinuous according to the 

 answer which this question may receive. 



SECTION IX. 



Discontinuity in Substantive Variation : Colour 

 and Colour-Patterns. 



From the consideration of Discontinuity in the Variation of a 

 character, size, which may be readily measured arithmetically, we 

 pass to the more complex subject of Discontinuous Variation in 

 qualities which are not at once capable of quantitative estimation. 

 In this connexion the case of colour-variation may be profitably 

 considered. Nature abounds with examples of colour-polymor- 

 phism, and in numerous instances such Variation is discontinuous. 

 Of such discontinuous Variation in colour I shall speak under two 

 heads, considering first variations in colours themselves and 



