III. SCHIZOPHYTA.
In this group are included small single-celled plants of an extremely low type of organisation, in which reproduction takes the form of multiplication by simple cell-division, or the formation of spores. The characteristic method of reproduction by division has given rise to the general term Fission-plants for this lowest sub-class in the vegetable kingdom. In many cases the members of this sub-class contain chlorophyll, and associated with it a blue-green colouring matter; such plants are classed together as the Blue-green algae, Cyanophyceae, or Schizophyceae. Others, again, are destitute of chlorophyll, and may be conveniently designated Schizomycetes or Fission-fungi. Seeing how close is the resemblance and relationship between the members of the sub-class, it has been the custom to include them as two parallel series under the general head, Schizophyta, rather than to incorporate them among the Algae and Fungi respectively.
A. SCHIZOPHYCEAE (Cyanophyceae or Blue-green Algae).
Chroococcaceae. Thallus of a single cell, the cells may be either free, or more usually joined together in colonies enveloped by a common gelatinous matrix, formed by the mucilaginous degeneration of the outer portion of the cell-walls. Reproduction by means of simple division or resting cells.
Nostocaceae. Thallus consists of simple or branched rows of cells in which special cells known as heterocysts often occur. Reproduction by means of germ-plants or hormogonia, or by resting cells specially modified to resist unfavourable conditions.
In both families the individuals are surrounded by a gelatinous envelope, which in some genera assumes the form of a conspicuous and comparatively resistant sheath. Marine, freshwater, and aerial forms are represented among recent genera. Several species occur as endophytes, living in the tissues or mucilage-containing spaces in the bodies of higher plants. In addition to the frequent occurrence of blue-green algae in freshwater streams and on damp surfaces, certain forms are particularly abundant in the open sea[176], and in lakes or meres[177] where they are the cause of what is known in some parts of the country as “the breaking of the meres” (“Fleurs d’eau”). From the narrative of the cruise of the Challenger, we learn that the Oscillariaceae are especially abundant in the surface waters of the ocean. The “sea sawdust” so named by Cook’s sailors[178], and the same floating scum collected by Darwin[179], affords an illustration of the abundance of some of these blue-green algae in the sea.
Another manner of occurrence of these plants has been recorded by different writers, which is of special importance from the point of view of fossil algae. On the shores of the Great Salt Lake, Utah, there are found numerous small oolitic calcareous bodies thrown up by the waves[180]. These are coated with the cells of Glœocapsa and Glœotheca, two genera of the Chroococcaceae. Sections of the grains reveal the presence of the same forms in the interior of the calcareous matrix, and it has been concluded, on good evidence that the algae are responsible for the deposition of the carbonate of lime of the oolitic grains. By extracting the carbonic acid which they require as a source of food, from the waters of the lake, the solvent power of the water is decreased and carbonate of lime is thrown down. In similar white grains from the Red Sea[181] there is a central nucleus in the form of a grain of sand, and cells of Chroococcaceae occur in the surrounding carbonate of lime as in the Salt Lake oolite. Prof. Cohn of Breslau in 1862 demonstrated the importance of low forms of plant life in the deposition of the Carlsbad “Sprudelstein[182].” On the bottom of Lough Belvedere, near Mullingar in Ireland[183], there occur numerous spherical calcareous pebbles, of all sizes up to that of a filbert. From a pond in Michigan (U.S.A.)[184] similar bodies have been obtained varying in diameter from one to three and a half inches. In the former pebbles a species of Schizothrix, one of the Nostocaceae occurs in abundance, in the form of chains of small cells enclosed in the characteristic and comparatively hard tubular sheath, and associated with Schizothrix fasciculata there have been found Nostoc cells and the siliceous frustules of Diatoms. In the Michigan nodules the same Schizothrix occurs, associated with Stigonema and Dichothrix, other genera of the Nostocaceae. One of the Michigan pebbles is shown in section in fig. 32 D.
OOLITIC STRUCTURE.
The connection between the well-known oolitic structure, characteristic of rocks of various ages in all parts of the world, and the presence of algal cells is of the greatest interest from a geological point of view. In recent years considerable attention has been paid to the structure of oolitic rocks, and in many instances there have been found in the calcareous grains tubular structures suggestive of simple cylindrical plants, which have probably been concerned in the deposition of the carbonate of lime of which the granules consist. In 1880 Messrs Nicholson and Etheridge[185] recorded the occurrence of such a tubular structure in calcareous nodules obtained from a rock of Ordovician age in the Girvan district of Scotland. These Authors considered the tubes to be those of some Rhizopod, and proposed to designate the fossil Girvanella.
Girvanella (fig. 26).
Messrs Nicholson and Etheridge defined the genus as follows:—
“Microscopic tubuli, with arenaceous or calcareous (?) walls, flexuous or contorted, circular in section, forming loosely compacted masses. The tubes, apparently simple cylinders, without perforations in their sides, and destitute of internal partitions or other structures of a similar kind.”
Fig. 26. Girvanella problematica, Eth. and Nich. Tubules of Girvanella lying in various positions and surrounding an inorganic ‘nucleus’ or centre. From a section of Wenlock limestone, May Hill. × 65
Since this diagnosis was published very many examples of similar tubular fossils have been described by several writers in rocks from widely separated geological horizons. The accompanying sketch (Fig. 26), drawn from a micro-photograph kindly lent to me by Mr Wethered of Cheltenham, who has made oolitic grains a special subject of careful investigation, affords a good example of the occurrence of such tubular structures in an oolitic grain of Silurian age from the Wenlock limestone of May Hill, Gloucestershire[186]. In the centre is a crystalline core or nucleus round which the tubules have grown, and presumably they had an important share in the deposition of the calcareous substance. The nature of Girvanella, and still more its exact position in the organic world, is quite uncertain; it is mentioned rather as à propos of the association of recent Cyanophyceae with oolitic structure, than as a well-defined genus of fossil algae.
In the typo description of the calcareous nodules from Michigan, Murray speaks of the Schizothrix filaments at the surface of the pebbles as fairly intact, while nearer the centre only sheaths were met with. It is conceivable that in some of the tubular structures referred to Girvanella we have the mineralised sheaths of a fossil Cyanophyceous genus[187]. The organic nature of these tubules has been a matter of dispute, but we may probably assume with safety that in some at least of the fossil oolitic grains there are distinct traces of some simple organism which was in all likelihood a plant. Some authors have suggested that Girvanella is a calcareous alga which should be included in the family Siphoneae[188]. As a matter of fact we must be content for the present to leave its precise nature as still sub judice, and while regarding it as probably an alga, we may venture to consider it more fittingly discussed under the Schizophyta than elsewhere.
Wethered[189] would go so far as to refer oolitic structure in general to an organic origin. While admitting that a Girvanella-like structure has been very frequently met with in oolitic rocks, it would be unwise to adopt so far-reaching a conclusion. It is at least premature to refer the formation of all oolitic structure to algal agency, and the evidence adduced is by no means convincing in every case. The discovery of Girvanella and allied forms in rocks from the Cambrian[190], Ordovician, Silurian, Carboniferous, Jurassic and other systems is a striking fact, and lends support to the view that oolitic structure is in many cases intimately associated with the presence of a simple tubular organism. Among recent algae we find different genera, and representatives of different families, growing in such a manner and under such circumstances as are favourable to the formation of a ball-like mass of algal threads, which may or may not be encrusted with carbonate of lime. Similarly as regards oolitic grains of various sizes, and the occurrence in rocks of calcareous nodules, the tubular structure is not always of precisely the same type, and cannot always be included under the genus Girvanella.
Several observers have recorded the occurrence of low forms of plant-life in the waters of thermal springs. It has been already mentioned that Cohn described the occurrence of simple plants in the warm Carlsbad Springs, and fission-plants of various types have been discovered in the thermal waters of Iceland, the Azores[191], New Zealand, the Yellowstone Park, Japan, India, and numerous other places.
A few years ago Mr Weed, of the geological survey of the United States, published an interesting account of the formation of calcareous travertine and siliceous sinter in the Yellowstone Park district[192]. This author emphasizes the important rôle of certain forms of plants in the building up of the calcareous and siliceous material. Among other forms of frequent occurrence, Calothrix gypsophila and a species Leptothrix are mentioned, the former being a member of the Nostocaceae, allied to Rivularia, and the latter a genus of Schizomycetes. In many of the springs there are found masses of algal jelly like those previously described by Cohn in the Carlsbad waters. Sections of such dried jelly showed a number of interlaced filaments with glassy silica between them. Weed refers to the occurrence of small gritty particles in this mucilaginous material. These are calcareous oolitic granules which are eventually cemented together into a compact and firm mass of travertine by the continued deposition of carbonate of lime. The presence of the plant filaments is often difficult to recognise in the “leathery sheet of tough gelatinous material,” or in “the skeins of delicate white filaments” which make up the travertine deposits.
BORINGS IN SHELLS.
Under the head of Cyanophyceae, mention should be made of the recent genus Hyella[193], which occurs as a perforating or boring alga in the calcareous shells of molluscs. On dissolving the carbonate of lime of shells perforated by this alga, the latter is isolated and appears to consist of rows of small cells, with possibly some sporangia containing spores. Other boring algae have been recorded among the Chlorophyceae, and recently a member of the Rhodophyceae[194] has been found living in the substance of calcareous shells. Such examples are worthy of note in view of the not infrequent occurrence of fossil corals, shells and fish-scales, which have evidently been bored by an organism resembling in form and manner of occurrence these recent algal borers.
The occurrence of small ramifying tubes in recent and fossil corals, fish-scales, and bones was long ago pointed out by Quekett[195], Kölliker[196], Rose[197] and other writers[198]. These narrow tubular cavities have generally been attributed to the boring action of some parasitic organism, either a fungus or an alga. In 1876 Duncan published two important papers[199] dealing with the occurrence of such tubes in recent corals, as well as in the calcareous skeleton of Calceolina, Goniophyllum and other Palaeozoic, Mesozoic and Tertiary species of corals. This writer attributed the formation of the cavities in the case of the fossil species to the action of a fungus which he named Palaeachlya perforans, and considered as very nearly related to Achlya penetrans found in the “dense sclerenchyma” of recent corals. In fig. 27 A. is reproduced one of the drawings given by Rose[200] in his paper published in 1855; it shows a section of a fish-scale from the Kimeridge clay which has been attacked by a boring organism. Rose attributes the dichotomously branched canals to some “infusorial parasite.”
Fig. 27. A, Section of a fish-scale from the Kimeridge Clay, showing branched canals, made by a boring organism, × 85. B, Section of a Solen shell, penetrated in all directions by the boring thallus of Ostracoblabe (a fungus?), × 330. C, Piece of the thallus of Ostracoblabe isolated by decalcification, × 745. A, after Rose. B and C, after Bornet and Flahault.
In the important paper by MM. Bornet and Flahault on perforating algae a full description is given of various boring forms belonging to the Chlorophyceae and the Cyanophyceae[201]. The canals which these algae produce in calcareous shells and other hard substances are of the same type as those previously described in fossil corals, fish-scales and bones. In dealing with living perforating Thallophytes the colour and other cell-contents often enable us to distinguish between algae and fungi, but in fossil specimens such tests cannot be applied. The fossil tubular borings may or may not show traces of the transverse septa and reproductive cells; it is often the case that no trace of the organism has been left, but only the canals by which it penetrated the calcareous or bony skeleton. In some of the examples of Palaeachlya figured by Duncan there appear to be numerous spores in some of the sections, but it is generally a very difficult and often an impossible task to discriminate between the borings of fungi and algae in fossil material.
Fig. 27 B, which is copied from one of Bornet and Flahault’s drawings, represents a piece of Solen shell riddled with small canals made by the organism which has been named by the French authors Ostracoblabe implexa, and regarded by them as a fungus. Fig. 27 C represents a small piece of the vegetative body of Ostracoblabe obtained from a decalcified shell. In endeavouring to determine the organism which has produced borings in fossil corals or shells, it must be borne in mind that some forms of canals or passages may have been the work of perforating sponges, but these are larger in diameter than those made by algae or fungi. By some writers[202] the tubular cavities in shells have been referred to true algae, but others consider them to be of fungal origin.
As an example of a fossil alga referred to the Cyanophyceae, the genus Zonatrichites[203] may be quoted. Bornemann, who first described the specimens, points out the close resemblance in habit to some members of the recent Rivulariaceae.
Zonatrichites.
The author of the genus defines it as follows:—
“A calcareous alga, with radially arranged filaments, forming hemispherical or kidney-shaped layers, growing on or enclosing other bodies. Parallel or concentric zones are seen in cross-section, formed by the periodic growth of the alga, the older and dead layers serving as a foundation on which the young filaments grow in radially arranged groups.”
The nodules which are apparently formed by species of this genus occur in various sizes and shapes; Bornemann describes one hemispherical mass 8 cm. broad and 4 cm. thick. In some cases the organism has given rise to oolitic spherules, which in radial section exhibit the branched tubular cells spreading in fan-shaped groups from the centre of the oolitic grain. The section parallel to the surface of a nodule presents the appearance of a number of circular or elliptical tubes cut across transversely or more or less obliquely. The resemblance between the fossil and a specimen of the recent species Zonatrichia calcivora Braun, is certainly very close, but it is very difficult, in the absence of material exhibiting more detailed structure than is shown in the specimens described by Bornemann, to decide with any certainty the true position of the fossil. The figures do not enable us to recognise any trace of cells in the radiating tubes. It is possible that we have in Zonatrichites an example of a Cyanophyceous genus in which only the sheaths of the filaments have been preserved. In any case it is probable that this Mesozoic species affords another instance of a fossil alga which has been responsible for certain oolitic or other structures in limestone rocks.
The species described by Bornemann was obtained from a Breccia near Lissau in Silesia, of Keuper age.
M. Renault has recently described certain minute structures in a Palaeozoic coprolite to which he gives the name Gloioconis Borneti[204], and which he regards as a Permian gelatinous alga similar to the well-known recent genus Glœocapsa. The appearances revealed in a section of the coprolite are interpreted by this author as a collection of small colonies of a unicellular gelatinous alga in various stages of development. Renault’s figure shows a spherical group of faintly outlined and cloudy bodies, most of which include one or two small dark spots. The latter are regarded as the cells of the alga, and the surrounding cloudy substance is described as the gelatinous sheath. The absence of a nucleus in these extremely minute fossil cells (8–10 µ in diameter) is referred to as an argument in favour of referring the organism to the Cyanophyceae rather than to the Chlorophyceae. It is possible that the ill-defined structure described by Renault may be a petrified alga, but there is not sufficient evidence to warrant a decided opinion; the absence of nuclei can hardly be taken seriously in such a case as this as an argument in favour of the Cyanophyceae.
CYANOPHYCEAE.
Although our exact knowledge of fossil Cyanophyceae is extremely small, it is probable that such simple forms of plants existed in abundance during the past ages in the earth’s history. Several writers have expressed the opinion that the blue-green algae may be taken as the modern representatives of those earliest plants which first existed on an archaean land-surface. The living species possess the power of resisting unfavourable conditions in a marked degree, and are able to adapt themselves to very different surroundings. Their occurrence in hot springs proves them capable of living under conditions which are fatal to most plants, and suggests the possibility of their occurrence in the heated waters which probably constituted the medium in which vegetable life began. An interesting example of the growth of blue-green algae under unfavourable conditions was recorded in 1886 by Dr Treub[205] of the Buitenzorg Gardens, Java. In 1883 a considerable part of the island Krakatoa, situated in the Straits of Sunda, between Sumatra and Java, was entirely destroyed by a terrific volcanic explosion. What remained had been reduced to a lifeless mass of hot volcanic ashes. Three years later, Treub visited the island, and found that several plants had already established themselves on the volcanic rocks. Various ferns and flowering plants were recorded in Treub’s description of this newly established flora. It seemed that the barren rocky surface had been prepared for the more highly organised plants by the action of certain forms of Cyanophyceae, which were able to live under conditions which would be fatal to more complex types.
In the petrified tissues of fossil plants there are occasionally found small spherical vesicles, with delicate limiting membranes, in the cavities of parenchymatous cells or in the elements of vascular tissue. Some of these spherical inclusions have been described as possibly simple forms of endophytic algae[206], such as we are now familiar with in species of the Cyanophyceae and other algae. So far, however, no recorded instance of such fossil endophytic algae is entirely satisfactory. Some of the cells figured by Williamson as possibly algae, endophytic in the tissues of Coal-Measure plants, are no doubt thin-walled vesicles which formed part of a highly vacuolated cell-contents. Examples of such vesicles in living and fossil cells are shown in fig. 42. The fact that the contents of living plant tissues have been erroneously described as endophytic organisms, should serve as a warning against describing fossil endophytes without the test of good evidence to support them.
The description of a fossil Nostoc by the late Prof. Heer[207] from the Tertiary rocks of Switzerland cannot be accepted as a trustworthy example of a fossil plant, much less of a genus of recent algae. The application of recent generic names to fossils which are possibly not even organic must do more harm than good.
B. SCHIZOMYCETES (Bacteria).
It is impossible to draw a sharp line between the two subdivisions of the Schizophyta. The so-called Fission-Fungi or Bacteria differ from the Schizophyceae or Fission-Algae in the cell-contents being either colourless, blood-red or green, but never blue-green. We may regard the Bacteria, generally, as the lowest forms of plants; they are extremely simple organisms which have been derived from some primitive types which possessed the power of independent existence and contained chlorophyll—that important substance which enables a plant to obtain its carbon first-hand from the carbon dioxide of the atmosphere.
Bacteria may be briefly described as single-celled plants, and as de Bary suggested comparable in shape to a billiard ball, a lead pencil or a corkscrew[208]. A single spherical or cylindrical cell measures about 1 µ in diameter[209]. They occur either singly or in filaments, or as masses of various shapes consisting of numberless bacterial cells. The nature and manner of life of Bacteria, and their extraordinary power of successfully resisting the most unfavourable conditions, render it probable that they constitute an extremely ancient group of organisms.
The wonderful perfection of preservation of many fossil plants enables us to investigate the contents of petrified cells and to examine in minutest detail the histology of extinct plants. To those who are familiar with the possibilities of microscopical research as applied to silicified and calcified fossil tissues, it is by no means incredible that evidence has been detected of the existence of Bacteria as far back in the history of the earth as the Carboniferous and Devonian periods.
Were there no trustworthy records of the occurrence of Bacteria in Palaeozoic times, it would still be a natural supposition that these ubiquitous organisms must have been abundantly represented. It has been suggested as a probable conclusion that some forms of Bacteria, which produced chemical changes in the soil necessary for the nutrition of plants, must have existed contemporaneously with the oldest vegetation[210].
The paper-coal of Toula, which in some places reaches a thickness of 20 cm., is a plant-bed of exceptional interest. It differs from ordinary coal in being made up of numberless thin brown-papery sheets associated with a darker coloured substance largely composed of ulmic acid. Prof. Zeiller[211], in an interesting account of the papery layers, has shown that they consist of the cuticles of a Lepidodendroid plant, Bothrodendron. An examination of a piece of one of the sheets at once reveals the existence of a regular network of which the walls of the meshes are the outlines of the epidermal cells, the meshes being bridged across by a thin light brown membrane which represents the layer of cuticularised cell-wall of each epidermal cell. At regular intervals and disposed in a spiral arrangement, we find small gaps in the papery cuticle which mark the position of the Bothrodendron leaves. These Palaeozoic cuticles are not petrified; they are only slightly altered, and have retained the power of swelling in water, being able to take up stains like recent tissues. It may reasonably be assumed that the persistent cuticles owe their preservation to a greater power of resistance to destructive agents than was possessed by the other tissues of the plant. It is by no means unlikely, as Renault[212] has recently suggested, that as the Bothrodendron stem-fragments lay in the swamps or marshes the tissues were gradually eaten away by Bacteria, but the cuticles successfully resisted the attacks of the bacterial saprophytes. The same observer has described what he regards as the actual organism which effected this wholesale destruction, under the name Micrococcus Zeilleri. He finds, after treating the cuticles with ammonia to remove the ulmic acid, that there occur numerous minute spherical bodies, each surrounded by a thin envelope, either singly or in groups on the surface of the cuticular membrane. These vary in size from ·5µ to 1µ in diameter. I have not been able to detect any satisfactory proof of such Micrococci in specimens of the paper-coal which were treated according to Renault’s method, but it is extremely probable that this unusual method of preservation of stem-cuticles is the result of selective bacterial action.
Renault believes that some of the minute spherulitic structures which are seen in sections of decayed tissues of Palaeozoic plants owe their origin, in part, to the ravages of bacteria. The disorganisation of parenchymatous cells gives rise to a gelatinous substance in which needle-like crystals of silica may be deposited, from a siliceous solution, in a matrix which has resulted from bacterial activity. In some of the sections of tissues figured by Renault[213] the outlines of a few cells are still indicated by fragments of the partially decayed wall, while in other cells the walls have been completely destroyed by Bacteria of which some are preserved in the centre of the cell-area, forming a kind of nucleus to the siliceous spherulites.
BACILLI.
In addition to the Micrococcus described by Renault from the Toula paper-coal, there are a host of other forms which have been minutely diagnosed and figured by Profs. Renault and Bertrand[214]. These authors have discovered what they believe to be well-defined species of Micrococcus and Bacillus ranging in age from Devonian to Jurassic. The material which has afforded the somewhat startling results of their investigations consists partly of the coprolites of reptiles and fishes, and of silicified and calcified plant tissues.
Bacillus Permicus. Ren. and Bert.[215] (Fig. 28 B.)
This Bacillus, which was discovered in sections of a Permian coprolite from Central France, has the form of cylindrical rods 12–14µ in length, and 1·3–1·5µ broad, rounded at each end. The rods occur either singly or occasionally, two or three individuals are joined end to end. Fig. 28 B represents a piece of one of Renault and Bertrand’s sections; the small rods are clearly seen lying in various directions in the homogeneous matrix of the coprolite. Each individual is said to be surrounded by an extremely minute empty space ·4µ in width, originally occupied by the Bacillus membrane, the central rod representing the mineralised cell-contents. In this example the petrifying substance was probably derived from the phosphate of calcium of bones which were attacked by Bacteria. I am indebted to Prof. Renault for an opportunity of examining specimens of this and other fossil Bacteria, and in this particular case there is undoubtedly strong evidence in favour of the author’s determination.
Fig. 28. A, Bacillus Tieghemi Ren. and Micrococcus Guignardi Ren. B, Bacillus Permicus Ren. (After Renault.)
Bacillus Tieghemi Ren.[216] and Micrococcus Guignardi Ren.[217] (Fig. 28 A.)
Renault has given the name Bacillus Tieghemi to certain minute rods 6–10µ, in length, and 2·2–3·8µ broad, often containing a dark coloured spherical spore-like body 2µ in diameter, which have been found in the tissues of a Coal-Measure plant.
The name Micrococcus Guignardi has been applied to more or less spherical bodies 2·2µ in diameter, also met with in silicified plants.
A portion of one of Renault’s figures is reproduced in Fig. 28 A. The faint and broken lines mark the position of the middle lamellae of parenchymatous cells from the pith of a Calamite. The tissue has been almost completely destroyed, but the more resistant middle lamellae have been partially preserved. The short and broad rods represent what Renault terms Bacillus Tieghemi; the small circle in the middle of some of these being referred to as a spore, and in one specimen shown in the figure, the second rod at right angles to the first is described as a small daughter-Bacillus formed by the germination of the central spore.
The isolated circles in the figure are referred to Micrococcus.
FOSSIL BACTERIA.
It is unnecessary to give an account of the numerous examples of Micrococci and Bacilli described by Renault from Devonian, Carboniferous, Permian and Jurassic rocks. We may, however, in a few words consider the general question of the existence and possible determination of fossil Bacteria.
In 1877 Prof. Van Tieghem[218] of Paris drew attention to the method of operation and plan of attack of Bacillus amylobacter as a destructive agent in the decay of plant débris in water. He was able to follow the gradual disorganisation of the tissues and the various steps in the ‘butyric fermentation’ effected by this Bacterium. Similarly the same author[219] was able to detect the action of an allied organism in some silicified tissues from the Carboniferous nodules of Grand-Croix, a well-known locality for petrified plants near Saint-Étienne. He recognised also the traces of the Bacillus itself in the partially destroyed plant tissues. The Palaeozoic Bacteria made use of some cellulose-dissolving ferment of which the action is clearly demonstrated in sections of silicified tissues. Many of the phenomena described by Renault and Bertrand as due to similar Bacterial action, afford additional evidence that the gradual disorganisation of vegetable tissues was effected in precisely the same manner as at the present day.
In some cases we have I believe trustworthy examples of the Bacteria themselves, both in coprolites and plant-tissues, but it is more than probable that some of the recorded examples are not of any scientific value. The examination of petrified tissues under the higher powers of a microscope often reveals the existence of numerous spherical particles and rod-like bodies which agree in shape with Micrococci or Bacilli. Minute crystals of mineral substances may occur in the siliceous or calcareous matrix of a petrified plant which simulate minute organic forms. Vogelsang[220] in his important work die Krystalliten has thrown considerable light on the ontogeny of crystals, and the minute globulites and other forms of incipient crystallisation might well be mistaken for Bacterial cells. Granting, however, that we have satisfactory evidence, both direct and indirect, that some forms of Bacteria lived in the decaying tissues of Palaeozoic plants, and in the intestines of reptiles and other animals, we cannot safely proceed to specific diagnoses and determinations[221].
Renault has pointed out that fossil Bacteria may often be more readily detected than living forms owing to the presence of a brown ulmic substance which results from the carbonisation of the protoplasm. He is forced to admit, however, that such diagnostic characters as are obtained by Bacteriologists by means of cultures cannot be utilised when we are dealing with fossil examples! We are told that “Partout où nous avons cherché des Bacteriaceés, nous en avons rencontré.”[222] This indeed is the danger; an extended examination of fossil sections under an immersion-lens must almost inevitably lead to the discovery of minute bodies of a more or less spherical form which might be Micrococci. To measure, and name such bodies as definite species of Micrococci is, I believe, but wasted energy and an attempt to compass the impossible.
Specialists tell us that the accurate determination of species of recent Bacteria is practically hopeless: may we not reasonably conclude that the attempt to specifically diagnose fossil forms is absolutely hopeless? “The imagination of man is naturally sublime, delighted with whatever is remote and extraordinary—”, but it is to be deplored if the fascination of fossil bacteriology is allowed to warp sound scientific sense.