THE PROBLEMS OF HEREDITY AND THEIR SOLUTION. 579 



We have now .seen the essential nature of the Mendel i;in prineiples 

 and are able to appreeiate the exact rehition in wliicli they stand to 

 the group of eases inchided in tlie law of aiu-estral luM-edity. In 

 seeking any general indieatioii as to the eonnnon properties of the 

 phenomena whieh aTe already known to obey Mendelian principles we 

 ean as yet point to none, and whether some sueli eonnnon features 

 exist or not is unknown. 



T'here is, however, one group of cases, delinite though as yet not 

 numerous, where we know that the Mendelian principles do not apply. 

 These are the phenomena upon which Mendel touches in his brief 

 paper on Hieraciuni. As he there states, the hy])rids, if they are fertile 

 at all, produce otfspring like themselves, not like their parents. In 

 further illustration of this phenomenon he cites Wichura's Salix hybrids. 

 Perhaps some dozen other such illustrations could be gi\'en which rest 

 on good evidence. To these cases the Mendelian principle will in 

 no wise ai)ply, nor is it easy to conceive any moditication of the law 

 of ancestral heredity which can express them. There the matter at 

 present rests. Among these cases, however, we perceive several more 

 or less eonnnon features. They are often, though not always, h^ybrids 

 between forms dift'ering in many characters. The lirst cross fre- 

 ({uently is not the exact interuK^diate ])etween the two parental types, 

 l)ut may, as in the few Hieracium cases, be irregular in this respect. 

 There is often some degree of sterility. In the al»sence of fuller and 

 statistical knowledge of such cases further discussion is impossil)le. 



Another class of cases, untouched ])y any hypothesis of heredity yet 

 propounded, is that of the false hybrids of Millardet, where we have 

 fertilization without transmission of one or several pai'ental characters. 

 In these not only does the first cross sliow, in some respect, the char- 

 acter or characters of one parent only, ))ut in its posterity no reap- 

 pearance of the lost character or characters is ()bser\ed. The nature 

 of such cases is still ({uite obscure, l)ut we luu'e to suppose that the 

 allelomorph of one gamete only develops after fertilization to the exclu- 

 sion of the corresponding allelomorph of the other gamete, nuich — if 

 the crudity of the com])arison may be pardoned —as occurs on the 

 female side in parthenogenesis without fertilization at all. 



To these as yet altogether unconformable cases we can scarcely 

 doubt that further experinuMit will add many more. Indeed, we 

 already have tolera])ly clear e\idence that many phenomena of inherit- 

 ance are of a much higher ordei- of complexity. When the paper on 

 Pisum was written Mendel apparently inclined to the view that with 

 modifications his law might be found to include all the phenomena of 

 hybridization, but in the brief subsequent paper on Hieracium he 

 clearly recognized the existence of cases of a difi'erent nature. Those 

 who read that contribution will be int(U'ested to see that he lavs down 



