Fig. 10. Gleichenia dicarpa Br. (1/2 nat. size.)
A brief reference may be made to another fern now represented by several species widely disseminated in tropical and sub-tropical countries. The genus Gleichenia occurs abundantly in the warmer regions of both the Old and New World. The fronds may usually be recognised by their habit of growth ([Fig. 10]); in several species the main axis is repeatedly forked and a small bud between the divergent branches of the forks forms a characteristic feature. The leaflets are either long and narrow like the teeth of a comb or short and bluntly rounded. Moreover the anatomy of the creeping stem affords a ready means of identification. We have satisfactory evidence of the occurrence of Gleichenia in European floras during both the Jurassic and Cretaceous periods. Numerous fragments of plants were obtained some years ago, not far from Brussels, from the Wealden strata in which the famous skeletons of Iguanodon were discovered. Visitors to the Natural History Museum in Brussels are no doubt familiar with the skeletons of this enormous herbivorous animal: in the same gallery are exhibited the remains of the fossil plants from the Iguanodon beds. Some of these fragments are pieces of fern fronds identical in form with those of existing Gleichenias. The microscopical examination of some exceptionally well preserved fragments of Wealden stems discovered by Prof. Bommer of Brussels enabled him to recognise the Gleichenia type of structure and thus to confirm the inconclusive evidence furnished by fragmentary leaves. The most interesting records in regard to the former occurrence of Gleichenia in Northern Europe we owe to the late Oswald Heer of Zurich, who has described many examples of Gleichenia fronds from rocks of Lower Cretaceous age in Disco Island on the west coast of Greenland in latitude 70° N. The same type of fern is recorded also from upper Jurassic beds in the north-east of Scotland, in the Wealden rocks of Sussex, as well as from other European localities. It is clear that the Gleichenia-family, no longer represented in north temperate floras, was in the Jurassic period, and especially in the early days of the Cretaceous period, widely spread in Europe, extending well within the Arctic circle. It may be that the original home of Gleichenia was in the far North at a time when climatic conditions were very different from those which now prevail. Gleichenia, like many other northern plants, retreated to more southern regions where, in the warmer countries of the world, many species still flourish widely separated in space and time from the place of their birth.
The ferns so far mentioned have a more or less extended distribution at the present day. In the case of Pteridium aquilinum, the cosmopolitan Bracken Fern, wide range would seem to be correlated with comparatively recent origin; on the other hand, the facts of palaeobotany show that the wide distribution of Osmunda, a type of fern which differs in many important respects from members of the family (Polypodiaceae) to which the Bracken belongs, is not inconsistent with an exceptionally ancient family-history. There are, however, certain genera of ferns which afford remarkable examples of restricted geographical distribution associated with great antiquity. The island of Juan Fernandez, 420 miles off the coast of Chili, the home for four years of Alexander Selkirk (to whose adventures we owe Defoe's creation of Robinson Crusoe), is interesting also from a botanical point of view. The vegetation of this oceanic island, 20 square miles in area with basaltic cliffs rising to a height of 3000 ft. above the sea, includes more than 40 species of ferns, eight of which occur nowhere else. One of these endemic ferns is Thyrsopteris elegans, the only representative of the genus; it is readily distinguished by its large and graceful fertile fronds, examples of which may occasionally be seen on a plant of this species in the Royal Gardens at Kew: the sporangia are produced in circular cups which replace the ordinary leaflets on the lower branches of the frond and hang from the short axis like miniature clusters of grapes. It is noteworthy that among the fragmentary remains of the fern vegetation of the Jurassic flora in England and in other parts of Europe specimens occur with fertile segments practically identical with those of the Juan Fernandez species. Students of fossil plants are occasionally led away by the temptation to identify imperfect specimens with rare existing species to which they exhibit a superficial resemblance, and this is well illustrated by the frequent use of the generic name Thyrsopteris for Jurassic and Lower Cretaceous ferns which are too imperfect to be determined with any degree of certainty We have, however, satisfactory grounds for the assertion that the Juan Fernandez fern affords a striking confirmation of the truth of Darwin's dictum that 'Rarity, as geology tells us, is the precursor to extinction.' In this remote oceanic island, for reasons which we cannot explain, there lingers an isolated type which belongs to another age.
The following passage, which forms a fitting introduction to an account of two other genera of ancient ferns, is taken from a description of an ascent of Mount Ophir in the Malay Peninsula by Dr A. R. Wallace in his well-known book on the Malay Archipelago:—'After passing a little tangled jungle and swampy thickets, we emerged into a fine lofty forest.... We ascended steadily up a moderate slope for several miles, having a deep ravine on the left. We then had a level plateau or shoulder to cross, after which the ascent was steeper and the forest denser till we came out upon the Padang-Batu, or stone-field.... We found it to be a steep slope of even rock, extending along the mountain side farther than we could see. Parts of it were quite bare, but where it was cracked and fissured there grew a most luxuriant vegetation, among which the pitcher plants were the most remarkable.... A few coniferae of the genus Dacrydium here first appeared, and in the thickets, just above the rocky surface, we walked through groves of those splendid ferns, Dipteris Horsfieldii and Matonia pectinata, which bear large spreading fronds on slender stems, 6 or 8 feet high'([49]).
Fig. 11. Matonia pectinata. A group of plants in a wood on Gunong Tundok, Mount Ophir. (Photograph by Mr A. G. Tansley.)
The two genera Matonia and Dipteris afford exceptionally striking examples of survivals from the past. Matonia is represented by two species, Matonia pectinata ([Fig. 11]), which grows abundantly on the upper slopes of Padang-Batu in dense thickets on the rock faces where, as Mr Tansley states, its associates are a species of Gleichenia, Dipteris, and a little Pteridium aquilinum (Bracken Fern). Matonia pectinata occurs also on Bornean mountains at an altitude of over 3000 ft. and descends to the coast on some of the Malay islands. The other species of the genus, Matonia sarmentosa, has so far been found in one locality only, Niak, Sarawak, where it was discovered by Mr Charles Hose. Matonia pectinata has a creeping stem covered with a thick felt of brown hairs bearing tall fan-shaped fronds divided into numerous comb-like branches thickly set with narrow linear leaflets on which circular clusters of spore-capsules are sparsely scattered. In some respects Matonia is unlike other ferns; the fronds constitute a striking feature, and the anatomy of the stem is still more distinctive. In the form, development, and arrangement of the sporangia (spore-capsules)—organs which from the constancy of their characters have long been recognised as the most useful basis for classification—Matonia exhibits distinctive features.
In order to emphasise the isolated position of the genus it has recently been placed in a separate family, the Matonineae, of which it is the sole living representative. The restricted geographical range of Matonia, considered in connexion with the clearly marked peculiarities in structure and form, leads us to expect other evidence in support of the natural inference that the genus is a survivor of a once more vigorous and widely spread family. If Matonia were a recently evolved type which has not spread far from its original home, we should expect it to conform more closely than it does to other ferns in the Malay region. Even assuming for the sake of argument that variation may occur per saltum, and new forms may be produced differing in more than the finer shades of small variation from their parents, the peculiar features of Matonia are too pronounced and its individual characteristics too obvious to warrant the assumption of recent production. It is, however, from the testimony of the rocks that we obtain confirmation of the opinion that these Malayan species are plants on the verge of extinction. In shales of Jurassic age exposed on the Yorkshire coast at Gristhorpe Bay and in iron-stained rocks of the same age between Whitby and Scarborough, well preserved leaves have been found agreeing in the shape of the frond, as also in the form of the leaflets and of the groups of sporangia which they bare, with those of Matonia pectinata.