history are frequently imperfect and occasionally missing, but others, again, are written in characters as clear as those which we decipher by a microscopical examination of the tissues of a recent plant. As one of the most striking instances in which the microscopic study of fossil plants has shown the way to a satisfactory solution of the problems of development, we may mention such extinct genera as _Lyginodendron_, _Myeloxylon_ and others. Each of these genera will be dealt with at some length in the systematic part of the book, and we shall afterwards discuss the importance of such types, from the point of view of plant evolution. The botanist who would trace out the phylogeny of any existing class or family, makes it his chief aim to discover points of contact between the particular type of structure which he is investigating, and that of other more or less closely related classes or families. Confining himself to recent forms, he may discover, here and there, certain anatomical or embryological facts, which suggest promising lines of inquiry in the quest after such affinities as point to a common descent. Without recourse to the evidence afforded by the plants of past ages, we must always admit that our existing classification of the vegetable kingdom is an expression of real gaps which separate the several classes of plants from one another. On the other hand our recently acquired and more accurate knowledge of such genera as have been alluded to, has made us acquainted with types of plant structure which enable us to fill in some of the lacunae in our existing classification. In certain instances we find merged in a single species morphological characteristics which, in the case of recent plants, are regarded as distinctive features of different subdivisions. It has been clearly demonstrated that in _Lyginodendron_, we have anatomical peculiarities typical of recent cycads, combined with structural characteristics always associated with existing ferns. In rare cases, it happens that the remarkably perfect fossilisation of the tissues of fossil plants, enables us not only to give a complete description of the histology of extinct forms, but also to speak with confidence as to some of those physiological processes which governed their life. [Sidenote: GEOLOGICAL HISTORY.] So far, palaeobotany has been considered in its bearings on the study of recent plants. From a geological point of view the records of ancient floras have scarcely less importance. In recent years, facts have been brought to light, which show that plants have played a more conspicuous part than has usually been supposed as agents of rock-building. As tests of geologic age, there are good grounds for believing that the inferiority of plants to animals is more apparent than real. This question, however, must be discussed at greater length in a later chapter.