THE MAIDENHAIR TREE — SEWARD 453 



single ancestral stock. In attempting to classify fossil plants on a 

 basis of natural affinity we have in most instances to use vegetative 

 characters alone; and this is the method followed in the comparison 

 of leaves of extinct plants with the foliage of recent species. There 

 are very few families of plants represented in present-day floras to 

 which only a single genus is allotted. Ginkgo is one of the few living 

 genera which has a family and indeed a whole group to itself. It 

 stands alone with no near relatives: if nothing were known of its 

 past history it would no doubt be regarded as an old type because of 

 the possession of certain primitive traits. Search among the debris 

 of forests embedded in the rocks has demonstrated that the genus was 

 formerly one of several distinguished by characters indicative of com- 

 munity of descent. In order to illustrate this aspect of the present 

 historical enquiry it is necessary to devote a few pages to a sketch of 

 the main results of paleobotanical research into the family history of 

 Ginkgo, without going into technical detail. 



The first genus to be considered is one known as Baiera: this name 

 (after J. J. Baier, an early eighteenth-century German writer on 

 fossils and minerals) was given as long ago as 1843 to leaves found 

 near Bayreuth in Rhetic or Lower Jurassic rocks. In outline the 

 leaves of Baiera are fan-shaped or triangular as in Ginkgo, but the 

 lamina is more deeply divided into linear segments, few or many in 

 number. It is not always possible to draw a satisfactory line be- 

 tween Baiera and Ginkgo from the form of the lamina alone. The 

 leaves of Ginkgo biloba and those of extinct species have a fairly long 

 and well-defined stalk: in those assigned to Baiera the leaf-blade 

 is attached directly to the branch by a narrow, tapered base and 

 lacks a leaf -stalk. In a typical Baiera leaf the blade is cut by deep 

 V-shaped sinuses into narrow, linear lobes or segments, each of which 

 is supplied with a few parallel and occasionally forked veins. Some 

 species bore the leaves in tufts on very short shoots; in others the 

 leaves were attached singly as in the long shoots of Ginkgo. It 

 may be that some Baieras had both long and short shoots. In the 

 structure of its epidermal cells, including the stomatal apparatus, 

 Baiera is near enough to Ginkgo to be included in the same family. 

 No undoubted example of Baiera has been found either in Tertiary 

 rocks or in those belonging to the later stages of the Cretaceous 

 period. A few species are recorded from the earlier Cretaceous beds, 

 but some at least of these might more appropriately be assigned to 

 Ginkgo. The genus was abundantly represented in both Jurassic 

 and Rhetic floras and in many parts of the world, reaching as far 

 north as lat. 70° N. in East Greenland. Some unusually large leaves 

 have been described from Upper Triassic beds in North America, 

 South Africa, and Australia. The oldest leaves believed to be ex- 

 amples of the genus are from Permian rocks. There is no doubt 



