144 POPULAE EEEOES. 



species became generally accepted, and the exist- 

 ence of this belief in no way interferes with the 

 present system of classification. This system of 

 classification may be compared to a tree, whose 

 larger branches, bearing limbs of smaller and still 

 smaller size correspond to the larger groups in the 

 vegetable kingdom, within which groups contain- 

 ing still smaller groups occur. It in no way 

 hinders the classification of the parts of a tree top 

 into limbs, branches, boughs and twigs for us to know 

 that every limb Was first a slender twig. Neither 

 does it interfere with the classification of plants 

 into greater and lesser groups, as we find them, 

 for us to believe that the present complex groups 

 have been developed from preexisting groups in 

 which the differences between the members was 

 less than they now are. How these differences 

 arose may be seen to some extent by the variations 

 arising in plants under cultivation. The cabbage, 

 kale, cauliflower and kohlrabi all originated from a 

 single wild species, which possesses none of the 

 special features of either of these cultivated plants. 

 It is a smooth-leaved plant, somewhat like the 

 mustard, destitute of a distinct head of any kind. 

 The cultivated varieties which have sprung from 

 this wild species, would, if they had been found 

 growing wild, have themselves been considered as 

 distinct species; but in practice it has not been 



