From a biological point of view considerable interest must always attend any investigation of the earliest known organisms, and, although we may legitimately infer that a flora and fauna existed prior to Cambrian times, the organisms preserved for us in rocks of that age constitute, at present, the first chapter of palæontological history. For this reason alone the Archæocyathinæ are important, since they form part of the Lower Cambrian fauna. When, however, we consider that the genera included in the group are very distinct from one another (indicating that the family was probably of considerable antiquity even in those early times), that the types have, as far as we know, a wide geographical distribution, and that to certain skeletal characters usually associated with the Porifera they unite others more common among the Cœlenterata, interest is still further stimulated. On the other hand, although recorded in great abundance from several widely separated localities, they are not, as a rule, common fossils in Cambrian strata, and consequently the group has not received much attention.

For over thirty years problems of stelar structure in the Ferns have claimed the attention of anatomists. By common consent protostely has been recognised as a primitive state. It is seen in all phyla of primitive vascular plants, except the semi-aquatic Equisetales, it figures universally in the juvenile plants of the Filicales, and is maintained in the adult stems of a number of their primitive genera.

But as to the origin of pith, inner phloëm, and inner endodermis, and the steps by which during descent the primitive protostely has in many instances been replaced in the adult by other more complex stelar states, there is no general agreement. On the one hand, the pith, inner phloëm, and inner endodermis have been regarded as purely intrastelar tissues directly referable in origin both in individual plants and descent to the procambial mass of the growing point of the stem.

Isle of Wight Bee Disease has been known in this country certainly since 1904, when it was first recognised in the island from which it derives its popular name. According to Imms it was probably present in Derbyshire in 1902, and was also known in Cornwall and other districts in 1904. Prior to these dates periodic losses of bees of a serious character are on record, dating as far back as the middle of the eighteenth century. Bullamore and Malden (1912) have summarised fully these outbreaks in historical series in their report in Journal of Board of Agriculture, Supplement 8, xix. From a study of the records which they have brought together and from personal inquiries which we have made at various bee-keepers of wide experience, it would appear that none of these earlier outbreaks attained the general distribution throughout the country which we know in Isle of Wight Disease at the present date, nor did any of them remain established over such an extensive period of years as that which has continued without interruption from 1902 until the present time.

Isle of Wight disease is, as we have seen, primarily a disease of the respiratory system, in which the organism remains localised throughout the entire course of the attack.

The effects are, however, far-reaching, and are registered in the disordered functioning of several organs, and in visible pathological changes in some of them.

The parasitic invasion has two aspects.

We have, in the first place, to consider the active injury wrought upon the host by a parasite developing and living at the expense of its body fluids. With this aspect of the question may be coupled the possibility of a definite toxic action on the part of the parasite.

In the second place, we have to consider the passive rôle of the mites in hindering or inhibiting the normal functions of the infected organs.

Before proceeding to consider the various pathological conditions, a few words on the distribution of the parasite within the host is called for.

The following experiments and observations have been undertaken with a view to discovering the means by which Tarsonemus is transmitted from one bee to another. It is obvious that a stage of the parasite exists outside the bee, and that there are also several possibilities (which may occur). One is the passage from bee to bee within the hive either directly or through the medium of frames or combs. Another, also within the hive, where the mites may in wandering upon the frames enter the cells and invade the body of the developing larvæ or pupæ, and in this way be present in the bee when it hatches. A third possibility is that whereby foraging infected bees may leave the mite upon flowers, vegetation generally, drinking grounds, or other situations, to be picked up later by other bees chancing to visit these. Crawling or dead bees may in a similar manner prove to be a source of infection through the contamination of the ground about the hive or of the actual hive itself.

The organism which has been found living in the anterior tracheal system of hive bees, and whose presence is associated with Isle of Wight disease, I have identified as a hitherto undescribed species of the genus Tarsonemus. This genus was founded in 1876 by Canestrini and Fanzago, and since then a moderate number only of species has been established. The true systematic position of these Acarines has been much in doubt, and their position in the order has from time to time been revised. Canestrini (1888) constituted the Tarsonemes the type of a special family, the Tarsonemini; they have been associated with the Oribatidæ by Berlese, and with the Cheyletidæ by Trouessart (1892). Banks (1904) regarded them as showing resemblances to the Tyroglyphidse, and placed them in a super-family Sarcoptoidea. An important character of the Tarsonemes is the existence of a tracheal system in the adult female, which is not found in the male nor in any pre-adult stage of either sex. This feature was adopted by Berlese (1897) as the basis of his sub-order, Heterostigmata, and by Oudemans (1906) in his division Trachelostigmata. This super-family includes two families—Tarsonemidæ and Scutacaridæ (Oudemans, 1916). This last is the Disparipedidae of Berlese.

Previous to the voyage of the Endurance in 1914–1915, the depth and extent of the Weddell Sea were either based on or surmised from the deep-sea soundings made on the Scotia by Dr Bruce in 1903 and 1904; on Dr Otto Nordenskjöld's ship Antarctic in 1901–1902; and on the German Expedition ship Deutschland under the leadership of Lieut. Filchner in 1912–1913. The first set are confined to the eastern and north-eastern portion, and may be said to run diagonally across the mouth of the sea; the second set were made in the extreme north-western area; whilst those of the Deutschland are disposed right down the centre of the sea, and consist mainly of a south-to-north series approximately along the meridian of 43° W.

The opportunities for observation were afforded by the voyage and subsequent drift of the S.Y. Endurance. During December 1914 and January 1915 for a period of six weeks she successfully bored her way through pack-ice of every description—drift-ice, open-pack, and very frequently even close-pack. Continually fighting, she penetrated from 59° to 72° S. lat., and finally reached the land water off Coats Land on the latter parallel. As the crow flies, therefore, she was navigated through ice for nearly 800 geographical miles on this voyage; her actual course among the ice-fields and floes was computed to exceed 2000 miles, an achievement without parallel in the Antarctic. The principle adopted was to keep to the east, where presumably there is less pack than in the west; if the Endurance experience is a normal one, however, the meridian of 20° W. long., which was followed, is certainly not far enough east.

This part of the account of our examination of the plants preserved in the silicified peat-bed of early Old Red Sandstone age found at Rhynie will be devoted to the consideration of a number of general questions concerning the four Vascular Cryptogams which it has yielded.

1. In the first place the morphological characters of Rhynia Gwynne-Vaughani, R. major, Hornea, and Asteroxylon will be reviewed and the attempt made to reconstruct the external appearance of these plants as they grew. In relation to this a few additional features of the plants will be described.

2. This will lead naturally to a consideration of the general bearings upon plant-morphology of the facts mentioned in this series of papers.

In this concluding part of the series of papers on the plants preserved in the Rhynie chert-band, various remains of lower plants that occur in the peat will be described, and some general questions concerning the accumulation and preservation of the deposit considered. The paper is divided naturally into the parts enumerated below.

1. An account will be given of a number of forms of Fungi which are met with in the decayed remains of the vascular plants, and also in the peaty matrix. The fungal remains are so generally distributed that they can be regarded as forming an integral part of the peat. In connection with them, the question as to whether there is evidence of a mycorrhizal relation between any of the Fungi and the Vascular Cryptogams will be considered.