CHAPTER XIV 



MISCELLANEOUS EXCEPTIONAL AND UNCONFORMABLE 



PHENOMENA. 



Crosses breeding true without Segregation. Parthenogenetic 

 or Apogamic Forms. Hieracium — Sexual Forms — 

 Nti'merical Aberrations — Irregularities of Dominance 

 — Alternation of Generations — Maternal Characters 

 in certain Seeds. 



Of the various cases alleged as exceptional, or declared 

 to be incompatible with Mendelian principles, few have 

 any authenticity. Several rest on errors of observation or 

 of interpretation and some have even been created by a 

 mistranslation or a misprint. With this class of exception 

 I have dealt on previous occasions. At a time when the 

 Mendelian idea was novel it was perhaps natural that a 

 doctrine so utterly at variance with received opinions should 

 have been regarded as suspect, and that from the conven- 

 tional standpoint the mere fact that a record was incom- 

 patible with Mendelism should have raised a presumption 

 in favour of its genuineness. That stage of the debate is 

 over, and we have now learnt that phenomena consistently 

 departing from Mendelian rules are so rare that announce- 

 ments of discoveries irreconcileable with the principle of 

 factorial composition may be safely disregarded, unless they 

 are made by observers experienced in this class of investi- 

 gation, or supported by evidence of exceptional strength. 



The progress of research has gone steadily to show that 

 facts of heredity which at first seemed hopelessly compli- 

 cated can be represented in terms of a strict Mendelian 

 system. This simplification of the problem has far exceeded 

 our earlier anticipations, and I have to regret that in 

 dealing with several sets of phenomena I countenanced 



