44 PROBLEMS OF GENETICS 



the bodies may be grouped in a common system of symmetry, 

 neither can be proved to show definite reversal of the parts. To 

 see that reversal recourse must be had to more extreme duplica- 

 tions, such as the famous Siamese Twins. They, as a matter of 

 fact, were an excellent instance of the proposition that twins 

 are related as mirror-images, for both of them had eleven pairs 

 of ribs instead of the normal twelve, and one of them had a 

 partial reversal of viscera. 6 (Kiichenmeister, Verlagerung, etc., 

 p. 204.) 



If anyone could show how it is that neither of a pair of 

 twins has transposition of viscera the whole mystery of division 

 would, I expect, be greatly illuminated. 7 At present we have 

 simply to accept the fact that twins, by virtue of their detach- 

 ment from each other, have the power of resuming the polarity 

 which is proper to any normal individual. It was nevertheless 

 with great interest that I read Wilder's recent observation 8 that 

 occasionally in identical twins the finger-print of one or both 

 the index-fingers may be reversed, showing that there is after 

 all some truth in the notion that reversal should occur in them. 



There is another phenomenon of twinning which, if we could 

 understand it, might help. I refer to the free-martin, the subject 

 of one of John Hunter's masterpieces of anatomical description. 

 In horned cattle twin births are rare, and when twins of opposite 

 sexes are born, the male is perfect and normal, but the repro- 



6 Mr. E. Nettleship tells me that in the course of collecting pedigrees of families 

 containing colour-blind members he has discovered two cases (shortly to be pub- 

 lished) of pairs of twins, which on account of their very close resemblances must 

 be deemed homologous, one of each pair being colour-blind and the other normal. 

 Such a distinction between closely similar twins is most cuiious and unexpected. 



7 Another paradoxical phenomenon of the same nature occurs in the Narwhal 

 The males normally have the left tusk alone developed, the corresponding right 

 tusk remaining as an undeveloped rudiment in its socket. The left tusk is a 

 left-handed screw. Occasionally the right tusk is also developed and grows to 

 the same length as that of the left side, but in such specimens the right tusk is 

 also a left-hand screw like the tusk of the other side, instead of being reversed 

 as we should certainly have expected. It need scarcely be remarked that in the 

 case of the horns of antelopes, and in other examples of spiral organs arranged in 

 pairs, that of one side of the body is the mirror image of that on the other side. 

 The Narwhal's tusks in being both twisted in the same direction are thus highly 

 anomalous, and are comparable with pairs of twins. 



8 Wilder, H. H., Amer. Jour. Anat., 1904, III, p. 452. 



