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VARIETIES 5 
semble each other and the parent plant in a great many re- 
spects; and these peculiarities which they all have in common 
they share to an almost equal extent with innumerable 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 by 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 resemblance 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, botanists 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 aspecial 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 
