256 Irregular Dominance [CH. 



nature of these imconformable families must be regarded 

 at present as altogether obscure. The records of human 

 polydactylism indicate that very similar disturbances of the 

 usual course of descent operate in regard to man. 



Another example almost exactly similar has been en- 

 countered both by Correns and by Lock in the heredity of 

 the blue or black colour of the aleurone layer of the seeds 

 of maize. The blue is generally a dominant appearing in 

 various degree, but some white F^ seeds proved able to 

 carry on the blue character. The problem raised by this 

 defect of dominance is discussed at length by Lock (174), 

 but no altogether satisfactory elucidation has been found. 



It may be pointed out that when, as in these examples, 

 the abnormal result is clearly perceptible in F^ no question 

 arises as to the occurrence of an imperfect segregation. 

 The peculiarity is evidently zygotic, and is caused either by 

 some feature of zygotic organisation, or by the influence of 

 external circumstances. This conclusion, so plainly dedu- 

 cible from results obtained in experimental crosses between 

 pure races, may be appealed to when, as in the descent of 

 some human characteristics, we witness as an exception 

 the handing down of a normally dominant factor through an 

 unaffected member of the family. In view of such occur- 

 rences the reality of the segregation may be doubted. No 

 experiment with pure types is possible in dealing with those 

 phenomena but the analogy of the genetics of extra toe in 

 fowls supports the view that the peculiarity of these cases is 

 zygotic and not due to failure of segregation. 



4. Alternation of Generations : an outstanding 

 difficulty. 



The question may be asked whether, on a general 

 survey of the facts of Natural History, any large classes 

 of phenomena are encountered which are altogether incon- 

 sistent with the Mendelian system to such a degree as to 

 suggest that they are quite incompatible with any scheme 

 of factorial analysis. Various exceptional cases have been 

 enumerated and considered. It is not in dispute that, for 

 example, in the phenomena of hybridisation between the 

 races of mankind, the facts (fairly well authenticated) must 



