8 The Problems 



Professor de Vries begins by reference to a remarkable 

 memoir by Gregor Mendel*, giving the results of his 

 experiments in crossing varieties of Pimm sativum. These 

 experiments of Mendel's were carried out on a large scale, 

 his account of them is excellent and complete, and the 

 principles which he was able to deduce from them will 

 certainly play a conspicuous part in all future discussions 

 of evolutionary problems. It is not a little remarkable 

 that Mendel's work should have escaped notice, and been 

 so long forgotten. 



For the purposes of his experiments Mendel selected 

 seven pairs of characters as follows :- 



1 . Shape of ripe seed, whether round ; or angular and 

 wrinkled. 



2. Colour of " endosperm " (cotyledons), whether some 

 shade of yellow ; or a more or less intense green. 



3. Colour of the seed-skin, whether various shades of 

 grey and grey-brown ; or white. 



4. Shape of seed-pod, whether simply inflated ; or 

 deeply constricted between the seeds. 



5. Colour of unripe pod, whether a shade of green ; or 

 bright yellow. 



6. Nature of inflorescence, whether the flowers are 

 arranged along the axis of the plant ; or are terminal and 

 form a kind of umbel. 



7. Length of stem, whether about 6 or 7 ft. long, or 

 about | to 1| ft. 



Large numbers of crosses were made between Peas dif- 

 fering in respect of one of each of these pairs of characters. 



* ' Versuche iib. Pflanzenhybriden ' in the Verh, d. Naturf. Ver. 

 Briinn, iv. 1865. 



