128 MENDELISM chap. 



of France with the paler British form, and found 

 that the offspring were more or less intermediate in 

 colour, and that in subsequent generations the parental 

 types did not recur. These cases at present stand 

 alone. It is possible that further research may- 

 reveal complications which mask or interfere with 

 an underlying process of segregation. Or it may 

 be that segregation does not occur owing to some 

 definite physiological reason which at present we do 

 not understand. 



And here it is impossible not to recall Mendel's 

 own experiences with the Hawkweeds {Hieracium). 

 This genus of plants exhibits an extraordinary pro- 

 fusion of forms differing from one another sometimes 

 in a single feature, sometimes in several. The 

 question as to how far these numerous forms were 

 to be classified as distinct species, how far as varieties, 

 and how far as products of chance hybridisation, was 

 even at that time a source of keen controversy among 

 botanists. There is little doubt that Mendel under- 

 took his experiments on the Hawkweeds in the hope 

 that the conception of unit-characters so brilliantly 

 demonstrated for the pea would serve to explain the 

 great profusion of forms among the Hieraciums. 

 Owing to the minute size of their florets, these 

 plants offer very considerable technical difficulties 

 in the way of cross-fertilisation. By dint of great 

 perseverance and labour, however, Mendel succeeded 

 in obtaining a few crosses between different forms. 

 These hybrids were reared and a further generation 

 produced from them, and, no doubt somewhat to 

 Mendel's chagrin, every one of them proved to breed 

 true. There was a complete absence of that segrega- 



