350 MEMSTIC VARIATION. [part i. 



pollex. But in a large majority of cases of the presence of an extra 

 digit on the radial side, the thumb has two phalanges as usual. Upon 

 a review of the evidence it is I think clear that we shall be right in 

 considering that in most of these cases the extra digit is not really in 

 Succession to the thumb, but that the two radial digits together repre- 

 sent the thumb, the increase in number being achieved by duplication 

 and not by successive addition. 



Most authors (Gruber, etc.) thus speak of these formations as 

 "double-thumbs" and recognize them as examples of duplicity, but it 

 should be remembered that this view of their nature is not consistent 

 with any statement that either of the two digits is the extra one. If 

 these thumbs are instances of duplicity then both together represent 

 the normally single thumb. 



In clear cases of double-thumb the two thumbs are equal or nearly 

 equal in size and development, as commonly happens in cases of true 

 duplicity. Double-thumbs are known in every degree of completeness. 

 The division between the two may occur at any point in their length. 

 Thus the duplicity may be confined to the nail and first phalanx 

 (Otto, Monstr. sexc. Descrip., Taf. xxv. fig. 1 ; Birnbaum, Monatsschr. 

 f. Geburtsk., 1860, xvi. p. 467); or it may include both first and second 

 phalanges (Gruber, Arch. f. path. Anat. Phys. x xxxn. 1865, p. 223); or 

 both phalanges and the greater part of the metacarpal (Gaillard, 

 Mem. Soc. de biol., 1861, p. 325); or even the whole digit and meta- 

 carpus, the two thumbs separately articulating with the trapezium 

 (Joseph, quoted by Gruber, I. c, p. 463, Note 37). It would be 

 interesting to know which of these conditions is the most frequent, for 

 it is likely that between the degrees of this variation there is Dis- 

 continuity, but the point is not easy to determine. As regards records 

 the conditions first and last named are much the rarest, and the double- 

 thumbs with two sets of phalanges articulating with one metacarpal 

 constitute the majority of cases. 



Sometimes the two thumbs are webbed together (Gruber, Bull. Ac. 

 Sci. Pet. xv. p. 480, fig.) sometimes they are separate and may be 



Fig. 101. Eight hand having a thumb double from the metacarpus, shewing the 

 relationship of images between the- two thumbs. (After Annandale.) 



