chap, xiv.] SYMMETRY IN DIGITAL VARIATION. 403 



the extreme forms of double-hand as seen in Man there is a curious 

 exception to this principle. For in nearly all the extreme cases 

 the abnormality was on one side only, the other being normal. 

 This was seen in Nos. 492 — 500 and 501 — 503, and also in Macucus 

 No. 504. The case No. 500 is probably an exception to this general 

 statement. As to the significance of this absence of correspond- 

 ence between the right and left sides in extreme cases of digital 

 Variation I can make no conjecture. It has seemed that perhaps 

 in such cases the absence of symmetry between the two sides 

 of the body may be connected with the fact that in these extreme 

 forms of double-hand an approach is made to a bilateral symmetry 

 completed within the series of digits. But against this suggestion 

 must be noticed first the fact that a similar bilateral symmetry is 

 established in the six-toed pes of the Cat (Condition IV of the pes, 

 p. 316), but the variation is nevertheless found on both sides of 

 the body ; and secondly the case of double-foot in the lamb (No. 

 566), though for reasons stated this latter case may perhaps be 

 open to question. 



(4) The manus and pes as systems of Minor Symmetry. 



This is a subject to which it is most difficult to give adequate 

 treatment. Several of the phenomena have as yet been studied in 

 far too small a range of cases to justify sound generalization, and 

 with further knowledge the suggestions arising from the facts now 

 before us may not improbably be found to have been misleading 

 wholly or in part. Besides this there is a serious difficulty in 

 finding modes of expressing with clearness even those principles of 

 form which seem to underlie the phenomena. This difficulty pro- 

 ceeds first from the vague and contradictory character of the 

 indications, and next from the total absence of a terminology by 

 which diversities of symmetry and the form-relations of parts may 

 be expressed. Nevertheless it has seemed best to abstain from 

 the introduction of new terms until the ideas to be expressed 

 shall have been more clearly apprehended. It need scarcely 

 be said that the remarks which follow merely represent an attempt 

 to state some of the lines of inquiry along which the facts point. 



On p. 88 mention was made of the fact that in a Bilateral 

 Symmetry the organs which occur as a pair, one on the right and 

 the other on the left, in so far as they are symmetrical are optical 

 images of each other, this relation of images being what is implied 

 by the statement that these organs are bilaterally symmetrical. 

 The hands and feet of vertebrates are organs of this kind, the 

 right hand and the right foot being approximately images of tin 

 left hand and foot respectively. But beyond their symmetrical 

 relations to each other in the Major Symmetry of the whole body 

 each manus and each pes may exhibit the condition of a Minor 

 Symmetry within the limits of its own series of digits. Not only 

 may each limb geometrically balance the limb of the other sid>- 



26—2 



