8. Species. Ordinarily, there is no danger of being misunderstood when we speak of such and such "sorts" or "kinds" of plants, in the way that people commonly do; but when we come to a careful study of plants we find among them such variety in the degrees of resemblance and difference that the necessity arises for a more precise means of expressing ourselves. It thus becomes important to understand something of the distinctions which naturalists recognize between the different degrees of likeness among living things.

When from a dozen seeds out of the same pod, say of a kidney-bean, we raise as many plants, there are twelve distinct individuals no two of which are exactly alike in all particulars. Yet despite their individual differences, they resemble each other and the parent plant in a great many respects; 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 offspring 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 connecting 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 influence, 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.