. The structure and life of birds . of classification has been arrived at. Unfor-tunately a system founded on such a basis, howevertrue it maybe, must always have this drawback—thatan amateur must accept a great deal of it on shall try to make clear a few of the chief points onwhich the best and more recent systems depend. Andthese few will be selected not only for their importancebut because they may be understood without muchtechnical knowledge. If they seem insufficient, itmust be remembered that there is a great array ofequally telling, but less easily appreciable, facets inreserve
. The structure and life of birds . of classification has been arrived at. Unfor-tunately a system founded on such a basis, howevertrue it maybe, must always have this drawback—thatan amateur must accept a great deal of it on shall try to make clear a few of the chief points onwhich the best and more recent systems depend. Andthese few will be selected not only for their importancebut because they may be understood without muchtechnical knowledge. If they seem insufficient, itmust be remembered that there is a great array ofequally telling, but less easily appreciable, facets inreserve. There are, as I have said, eleven thousand knownbirds. The first step is easy : they can be divided XV CLASSIFICATION 383 into two sub-classes. One is made up of those whichhave keels to their breastbones, the other of thosewhich have a rounded breastbone with no keel. Theformer are called Carinatae {carina — a keel] ; thelatter, Ratitae vatis = a keelless boat). This separatesoff the Apteryx, and the Ostrich with its kin the. Fig. 75.—Sternum of Ostrich. Rhea, the Emeu and the Cassowary. Thus some tenspecies out of the eleven thousand are disposed of. I must now mention a few of the points that arcmost useful for deciding to which order one of theCannate belongs. Very important are the followingquestions: the presence or absence of the first toe,and the arrangement of the four toes when all are 384 THE STRUCTURE AND LIFE OF BIRDS chap. present ; the relation of the branched tendon thatflexes the toes to that which flexes the hallux (seep. 167) ; the presence or absence of the ambiens muscle(see p. 169) which passes from the pelvis to the toes ;the distribution of the feathers on the neck and back—whether there is a clearly defined tract of featherson the neck with bare spaces or apteria on either side,and whether this tract forks, an apterion dividing it
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