156 MENDELISM chap. 



spring. Whether this may turn out to be the true 

 account or not, the possibility must not be over- 

 looked in future work. 



As a third example of an unlooked-for complica- 

 tion, we may recall Mendel's own experiences with 

 the Hawkweeds {Hzeraciuin). This genus of plants 

 exhibits an extraordinary profusion of forms differing 

 fro:n 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 litth doubt that Mendel undertook 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 

 fcA^ 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 segregation of characters 

 which he had shown to exist in peas and beans, 

 and had probably looked forward with some 

 confidence to finding in Hzeraciuin. More than 

 thirty years passed before the matter was cleared 

 up. To-day we know that the peculiar behaviour 

 of the hybrid Hieraciums is due to the fact 



