The progress of Genetics since the rediscovery of Äleuders papers. 399 



(ipinion in any case so cogent that it can be accepted without hesi- 

 tation, and if genuine examples exist, they must be exceedingly rare, 

 for I do not know of any that have been attested by adequate evi- 

 dence since the modern methods of genetic research have been in use. 



There are nevertheless the famous cases which Mendel himself 

 discovered in Hieracium, where the F, hybrids produce seeds which 

 repeat the hybrid type. The reseai'ches of R a u n k i a e r and Osten- 

 feld have however proved that the seeds in these cases arise by 

 parthenogenesis, and consequently the reproduction is really of an 

 asexual nature. It is by no means impossible that the same account 

 will be found applicable to some other cases also. 



The races of human mulattoes are often referred to as examples 

 of failure of segregation. Critical information respecting the inheri- 

 tance of mulattoes has, however, so far as I am aware, never been 

 collected. The imperfect indications that do exist suggest that the 

 half-breeds between Europeans and coloured races are not by any 

 means alike in their inheritance, and I have received information, 

 especially with regard to crosses with Indian races, which is distinctlj' 

 suggestive of some segregation. Undoubtedly the number of factors 

 involved must be large, but the question is whether it is indefinitely 

 large. Pending the collection of proper evidence however the case 

 of mulattoes must of course be regarded as constituting a definite 

 exception to the ordinary rules of segregation. 



The "pure lines" of Johaniisen. 



Among the many questions raised by genetic research that of 

 the magnitude of the units concerned is one of the most important. 

 As to their upper limits we are getting much evidence, but we do 

 not yet know at all how small they may be while still retaining their 

 definiteness. According to the views which Johannsen has based 

 on his remarkable researches on the weights of Phaseolus beans, these 

 definite units may be very small indeed. In this case, and in that 

 of the peculiar failure to seed {Scliariig'kdt) affecting a variable number 

 of florets in Barley. Johannsen (67) has detected the existence of 

 a great number of pure types or "lines", which, though differing from 

 each other in small quantitative degrees, he regards as definite 

 entities. This line of inquiry is of a specially interesting nature, 

 but it is obvious that in order to establish the definiteness of the 

 units involved, evidence of segregation will have to be found. Such 

 evidence in the case of quantitative characters, liable presumably to. 

 great fluctuations owing to diversity of external conditions, must be 

 peculiarly difficult to obtain. 



Progressas rei botanicae I. 26 



