VARIETIES 5 



somble each other and the parent plant in a great many re- 

 spects; and these pecuHarities which they aU have in common 

 they share to an ahnost (H]ual extent with innumeral)le other 

 individuals. Taken together all these individuals which 

 have this essential likeness form what is called a species. 

 Thus, a species is a group of individuals regarded hy experts 

 as having about the same degree of resemblance as parent and 

 offspring. 



It is, as we have seen, a familiar fact that among the off- 

 spring of a single individual there are commonly various 

 degrees of reseml)lance to the parent. The result is a more 

 or less complete series of intermediate forms connecting the 

 most dissimilar individuals one with another. Since among 

 individuals known to be related closely such intermediate 

 series commonly occur, l)otanists assume whenever connect- 

 ing links of this sort are found between more or less dissimilar 

 forms that the whole chain is so closely akin as to belong to 

 one species. A striking instance of widely different forms 

 connected closely by intermediate ones is afforded by the 

 various sorts of cabbage and their kin (Figs. 63-70). These 

 forms, including kale, cauliflower, kohlrabi, and Brussels 

 sprouts, were doubtless derived, largely under man's influ- 

 ence, from the wild kale. Accordingly the name Brassica 

 oleracea is applied not only to the wild plant but to all its 

 cultivated descendants. There is no general English name 

 applying to all the forms of this species. 



9. Varieties. We have seen above that among the off- 

 spring of a single plant there will be minor differences, and 

 that among the individuals of a species the differences may 

 be very considerable. If in the examination of a number of 

 specimens belonging to one species, a botanist finds certain 

 individuals possessing in common some peculiarity or set of 

 peculiarities which distinguish them clearly from the rest, 

 as, for example, cauliflowers in contrast with other plants of 

 the species Brassica oleracea, he calls the individuals thus dis- 

 tinguished a variety, and gives it a special name. The cauli- 

 flower thus becomes Brassica oleracea variety botrytis. Or, to 

 take an example from wild plants, there are found among the 

 individual trees that comprise the species called Salix nigra 



