[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 published) 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 curious 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. [9] Polydactylism which is often a dominant and the web-foot of Pigeons which is recessive should be remembered as possible exceptions (see p. 49). [10] Davenport inclined at first to regard rumplessness as a recessive, but in his latest publication on the subject he definitely concludes that it is an imperfect dominant. This conclusion accords well with evidence quoted by Darwin (_An. and Plts._, II, ed. 2, p. 4) that rumpless fowls may throw tailed offspring. (_Amer. Nat._, 1910, XLIV, p. 134.) [11] Spillman, W. J., _Amer. Breeders Mag._, 1910, I, p. 178. [12] Newsholme, _Lancet_, December 10, 1910, p. 1690. [13] _Materials for the Study of Variation_, 1894, p. 358. [14] Walker, G., _Johns Hopkins Hospital Bulletin_, XII, 1901, p. 129. [15] Cp. R. H. Compton, _New Phytologist_, 1911, p. 249. [16] _Arch. f. Entwickelungsmech._, 1907, XXIII, p. 249. [17] Bull. Soc. Bot. de France, xxxiv, 1887, p. 182. [18] R. Boyle, _The Origine of Formes and Qualities_, Oxford, 1666. [19] Remarkable experiments on this question have lately been carried out by R. H. Compton (_Camb. Phil. Soc._, XV, 1910, p. 495), showing that in a certain Barley, "Plumage Corn," the average ratio of left to right is about 1.5. A fuller paper has since been published by Compton, _Jour. Genetics_, 1912, II, I, p. 53. CHAPTER III SEGMENTATION, ORGANIC AND MECHANICAL