322 Appendixes 



doubleness in Stocks is approximately represented. Taking 

 X and Y as two factors, either of which is sufficient to 

 make the flower single, and representing the ever-sporting 

 type of single as XxYy, it is shown that the inheritance 

 from such a plant is given by a scheme in which the ovules 

 are arranged in a system 



iSXY: iXy : ixY i$xy 



and \hzpollen is all xy. 



To represent the descent from such a plant, if it be also 

 heterozygous for white and cream plastids, much more 

 complex expressions are provided. In these it is suggested 

 that the factors X, Y, and W (the factor for white plastids) 

 or any two of them may behave as inseparable in certain 

 ovules, and as separable in others. In the light of the new 

 facts as to coupling and repulsion this representation should 

 be regarded as provisional only. (For the evidence see 

 Saunders, E. R., Jour. Gen., 1911, i. p. 303.) 



In the same Journal, i. 1910, p. 57, Miss Saunders 

 describes experiments with Petunia, which show that the 

 pollen of singles is always single so far as has been deter- 

 mined, but the ovules of singles, of whatever source, consist 

 of a mixture of singles and doubles. Thus, any mixture 

 fertilised by a double, gives a mixture of singles and doubles, 

 but the singles thus produced will not give doubles, whether 

 self- fertilised or crossed inter se. It is not as yet clear why 

 the interbreeding of singles should give no pure singles, 

 but that nevertheless seems to be the fact. 



Appendix to Chapter XII. 



In regard to inheritance of sex-limited conditions, see 

 Appendix to Chap. X. 



The whole subject of the inheritance of human diseases 

 and malformations is now being rapidly explored, and for 

 extensions of knowledge on this branch of inquiry the 

 reader should consult the Treasury of Human Inheritance, 

 and especially the Transactions of the Ophthalmological 

 Society, in which the papers of E. Nettleship appear. 

 Among the latter is a collection of evidence as to hereditary 

 nystagmus (ibid., xxxi. 1911), a condition following lines 

 of descent similar to those of colour-blindness. 



