In the same way we find with our meadow plants that what we at first thought was a single kind, "_the_" buttercup really bears a name applicable to a genus in which are several common species. The genus is called Ranunculus, and there are several common English species with yellow flowers, but distinguished from one another by definite characters. They are Ranunculus acris, Ranunculus flammula, Ranunculus bulbosus, Ranunculus arvensis, Ranunculus ficaria (the lesser celandine). And then there is the white-flowered Ranunculus aquatilis–a common pond plant. Clover, again, is by no means the name for a single species. The clovers form the genus Trifolium, and in any English meadow we may come across the white clover, Trifolium repens; the red clover, Trifolium pratense; the hop clover, Trifolium agrarium: the strawberry clover, Trifolium fragiferum; the haresfoot clover, Trifolium arvense. So it is with the plants which at first sight we distinguish merely as "daisies." There are several distinct genera of daisies–Aster, Bellis, Chrysanthemum (ox-eye), Anthemis (camomile), and others, with several distinct species in each genus. Enough has been said to show the reader that the mere notion of "kinds" does not carry the same meaning as "species," but that there are a number of regularly occurring definite forms of both animals and plants which can be arranged in groups consisting only of individuals which are very nearly identical with one another. A group of living things of this degree of likeness is called "a species," and receives a name. A less degree of likeness holds together a number of species to form what we call a genus, and the name of the genus is cited together with the name of the species when we wish to speak of the species with clearness and certainty. This system of double names we owe to the great Swedish naturalist of the eighteenth century, Linnæus. He proposed also that the relationships of living things to one another should be further expressed by grouping like genera into "families," then like families into "orders," and like orders into "classes." And since his day we go further and group classes into "phyla" or great stems of the animal pedigree. In this way a complete hierarchy or system of less and more comprehensive groups has been established, and is the means by which we indicate the natural groups of the family-trees of plants and of animals, what, in fact, is called the "classification" of each of these great series of living things. Linnæus compared his system of groups to the subdivisions of two armies. Thus, the one army represents the whole animal series, the other the whole vegetable series. An army is divided into (1) "legions," these into (2) "divisions," "divisions" into (3) "regiments," regiments into (4) battalions, and battalions consist of (5) companies, consisting of individual soldiers. According to Linnæus, we may compare the legions to classes, which are divided into orders,