14 The Problems 



remembered that the principle is only explicitly declared 

 to apply to discontinuous characters*. As stated also 

 it can only be true where reciprocal crossings lead to the 

 same result. Moreover, it can only be tested when there 

 is no sensible diminution in fertility on crossing. 



Upon the appearance of de Vries' paper announcing the 

 " rediscovery " and confirmation of Mendel's law and its 

 extension to a great number of cases two other observers 

 came forward almost simultaneously and independently 

 described series of experiments fully confirming Mendel's 

 work. Of these papers the first is that of Correns, who 

 repeated Mendel's original experiment with Peas having 

 seeds of different colours. The second is a long and very 

 valuable memoir of Tschermak, which gives an account of 

 elaborate researches into the results of crossing a number 

 of varieties of Pisum sativum. These experiments were in 

 many cases carried out on a large scale, and prove the 

 main fact enuntiated by Mendel beyond any possibility of 

 contradiction. The more exhaustive of these researches 

 are those of Tschermak on Peas and Correns on several 

 varieties of Maize. Both these elaborate investigations 

 have abundantly proved the general applicability of Mendel's 

 law to the character of the plants studied, though both 

 indicate some few exceptions. The details of de Vries' 

 experiments are promised in the second volume of his most 

 valuable Mutationstheorie. Correns in regard to Maize 

 and Tschermak in the case of P. sativum have obtained 

 further proof that Mendel's law holds as well in the case of 

 varieties differing from each other in two pairs of characters, 

 one of each pair being dominant, though of course a more 

 complicated expression is needed in such cases f. 



* See later. 



f Tschermak's investigations were besides directed to a re-exami- 



