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The world of chemical phenomena is so immensely wide, and the phenomena themselves are so complicated, that the founders of modern chemistry were compelled to limit the area of their investigations, and sharply to separate their own domain from those of the two sister-sciences, physics and mechanics, leaving it to the future to find out the bonds which might unite all three branches into one harmonious whole. They and their followers elaborated their own methods of investigation; they discovered their own chemical laws and worked out their own hypotheses and theories; and, with the aid of these methods, laws, and hypotheses, they created a science which not only interprets, discovers, and predicts the phenomena it deals with, but already has brought us within a measurable distance of a general theory of the structure of matter altogether.
In proportion as chemical research went deeper into the study of the wonderful movements and interactions of molecules and atoms, the intimate connection which exists between chemistry, physics, and mechanics became more and more apparent. The physical and the chemical properties of matter proved to be so closely interdependent that they could be explained no longer with the aid of chemical theories alone; the very fundamental laws of chemistry appeared to be but so many expressions of physical facts; and chemistry stands now in such a position that no further advance in its theoretical part is possible, unless it enters the borderland which separates it from physics, recognises the unity of chemical and physical forces, and, availing itself of the progress recently made in molecular mechanics, boldly attacks the great problem of a physical—that is, a mechanical—interpretation of chemical facts. This is the work which now engrosses the attention of most chemists.
The points of contact between physics and chemistry are very numerous, and the work is being carried on in several directions at once. The discovery by Mendeleeff of the so-called 'periodical law of elements' has called into life numerous researches, some of which accumulate correct numerical data to express the dependence between the physical properties of various bodies and their chemical constitution; while others endeavour to interpret this very periodicity in the properties of the elements under the assumption of their compound nature. On the other side, the recent development of the mechanical theory of heat, and the interest awakened of late in electricity, have given rise to numerous researches aiming at a representation of chemical reactions as mere transformations of heatenergy or electricity. And, finally, most skilful investigations are being made, and most suggestive hypotheses advanced as regards the possible distribution of atoms within the molecules, under the supposition of their remaining in a state of equilibrium; and thus the way is prepared for a higher conception of the atoms—not motionless and mutually equilibrated, but involved, like the planets of our solar system, in complicated movements within the molecules. Works of importance have appeared of late in each of these directions. But no other domain has lately been explored with such a feverish activity as the vast domain of solutions; and to these researches we must now turn our attention.
In former times, it was supposed that if some table-salt, or sugar (or any other solid, liquid, or gas) is dissolved in water or in any other liquid, the particles of the dissolved body will simply spread, or glide, between the particles of the solvent, and simply be mixed together—just as if we had made a mixture of two different powders or two gases. But on a closer study a succession of most complicated and unexpected phenomena was revealed, even in so simple a fact as the solution of a pinch of salt in a tumbler of water. The solutions proved to be the arena upon which phenomena cease to be purely physical, and become chemical, and they were studied accordingly with the hope that they might give a physical cue to chemical reactions. Hundreds of researches are contributed every year to this subject;<ref>The Committee appointed by the British Association for reporting on the bibliography of solutions had catalogued no less than 255 papers, which appeared in 1890, in a few periodicals only. The total was at that time 930 papers.</ref> and although there is yet no final result to record, we are bound nevertheless to examine the present state of investigations which so much interest and excite chemists.<ref>We know no general review of this extremely complicated question which we might recommend to the general reader. The address delivered by Prof. Orme Masson before the Australasian Association fo> the Advancement of Science, in January 1891; Prof. 8. U. Pickering's Beport to the British Association', in 1890, on the hydrate theory of solution, followed by a most interesting discussion between Profs. Gladstone, Arrhenius, Armstrong, Fitzgerald, Van't Hoff, Lodge, Ostwald, and Bamsay, and the elaborate report, by W. N. Shaw, on electrolysis (British AsiociationBeportt, 1890, Leeds), are excellent sources of general information. Ostwald's work, Solutiont (English translation in 1891), as well as his Lehrbnch der allgemelncn Chemie (Leipzig, 1886; new edition of first volume in 1892), and the review, Zelttohrift fwr pkytihalitche Chemie, which he publishes since 1887, unhappily take but little notice of the chemical aspects of the question. Mendeleeff 's footnotes in his most remarkable Principles of Chemistry (London, 1891) are perhaps, on the whole, the best means for gaining a general and impartial insight into the whole question. Though himself one of the earliest promoters of the hydrate or chemical theory of solutions, he fully recognises the importance of the physical theories, and sums them up with his usual clearness.</ref>
Few scientific hypotheses have proved so productive in the development of science altogether as the so-called 'kinetic theory of gases.' A gas, according to this hypothesis, is an aggregate of molecules which move very rapidly in all directions and endeavour to disperse in space—the rapidity of their movements being increased by every increase of the temperature of the gas. In their endeavours to escape in all directions the molecules of the gases continually bombard the walls of the vessels which contain them. They break them if they are weak enough, or else they exercise upon them a pressure which is nothing but the sum of all energies of the molecules which strike a unit of surface in a unit of time. In our steamengines the molecules (or rather particles) of steam bombard the walls of the cylinder; they push the piston by their aggregate energies and, setting it in motion, make it move the huge masses it has to move. This is, of course, but an hypothesis; but since it so perfectly explains the pressure, the elasticity, the diffusion, and the internal friction of gases, and permits us to predict the consequences of the invisible bombardment; and since its consequences, mathematically deduced by Maxwell, Clausius, Boltzmann, and many others, fully agree with the reality of facts—it can be considered no more as a mere guess: it is a theory.
Now, the Dutch chemist Van 't Hoff proved in 1886 that the same theory holds good for weak solutions as well. If some sugar, or some sulphuric acid, or any other liquid or solid, be dissolved in some liquid, the bonds which keep together the particles of sugar or of the acid are torn asunder by the solvent. The particles spread among those of the solvent, and they take up the same movements which they would perform if the sugar or the acid were brought into a gaseous state in a free space. They bombard the walls of the vessel, and exercise upon them a certain pressure which will be increased if the bombardment is rendered more violent by either raising the temperature of the solution, or increasing the number of bombarding particles by a limited increase of its strength. Though there is not the slightest reason for supposing that the dissolved solid or liquid may be in a gaseous state within the solvent, the very fact of scattering its particles over a broad space is sufficient to free them from their mutual bonds; they behave exactly as if the sugar or the acid were brought into a gaseous state by evaporation and filled the space occupied by the solution. They obey all the physico-chemical laws (the laws of Boyle, Marriotte, Gay Lussac, and Avogadro) which hold good for gases.
The kinetic theory of gases was thus extended to liquids, and this first step was soon followed by another, even more important step, when Van der Waals—also a Dutch chemist—still more effectively bridged over the gap between the gaseous and liquid condition of matter. He studied that state of a gas when, under an increasing pressure and a decreasing temperature, it becomes a liquid; and he found a mathematical expression (an equation) which very approximately represents the mutual dependence between the volume occupied by the gas under a given pressure, its temperature, the volume occupied by its particles, and their mutual pressure. He thus expressed in a more comprehensive way how, in proportion as the lengths of the paths of its particles decrease, a gas becomes a liquid.<ref>See the interesting discussions which took place upon this subject in the Physical Society, in October and November last.</ref>
The long-since suspected continuity between the gaseous and liquid states of matter was thus demonstrated once more, and rendered easy to investigate; and the importance of these conclusions was still more enhanced by Clausius when he demonstrated that a slight alteration of Van der Waal's equation makes it also represent the absorption or dissipation of heat-energy which always takes place when a body passes from the liquid to the gaseous state, or vice, versa.
And, finally, another step in the same direction was made by the French physicist, Raoult. We all know that if some table-salt, or saltpetre, or some other salt, be added to water, the water may be cooled below zero without freezing. Its freezing temperature is lowered. Now Raoult studied the lowering of this temperature caused in water and other liquids by the addition of various amounts of various salts, and he came to a most remarkable result. It appeared that, whatever the nature of the dissolved salt may be, the freezing temperature of a solution will always be lowered by the same amount (nearly six-tenths of a degree) if we add one molecule of the dissolved body to each hundred molecules of the solvent.<ref>Thus, if table-salt be used, the weight of its molecule (compared to a molecule of hydrogen) is 58J; while the weight of a molecule of water (also compared with hydrogen) is 18. So ttat, if we add 58) ounces of table-salt to each 1,800 ounces of water, we shall lower its freezing temperature by 0-62° of the centigrade scale. The same result will be obtained if we take 74^ ounces of potassium chloride, or 101 ounces of saltpetre, to the same amount of water.</ref> Thus, again, a purely physical fact, such as freezing, proves to be dependent upon a purely chemical fact—the molecular weights of the solvent and the dissolved body; and this physical law is so general that it has become a very accurate means for determining such chemical data as molecular weights. Chemistry and physics appear again so closely interwoven that there is really no means of separating them.
It is not possible to describe in a few words the impetus given by the discovery of these connections to physico-chemical research altogether. A school, headed by Ostwald, of most enthusiastic supporters of what has been termed (not quite properly) the physical theory of solutions, has grown up; and this school, while bringing out a mass of important researches and widening the field of chemical investigations, has naturally come to consider itself as being on the right track for elaborating a complete theory of the subject. Unhappily, this is not the case, because the chemical reactions which undoubtedly take place in solutions are not taken into account in the just-mentioned physical laws. In reality, so long as but small amounts of solids, or liquids, or gases are dissolved in a liquid, and so long as only such bodies are brought into contact as have no strong chemical affinity to each other, the above theories are quite correct. But as soon as the solution is rendered stronger, or the solvent and the dissolved body are endowed with a mutual chemical affinity, chemical reactions set in. Part of the molecules of the dissolved body dissociate, and the atoms of which they were composed, on being set free, combine with the atoms of the solvent. Chemical forces, much more energetic than the physical forces, enter into play, and most complicated chemical reactions—the intensity of which may be judged of from the changes of temperature—begin. To deny them is simply impossible, although this has been done in the excitement of polemics. The chemical reactions which take place within the solutions, and especially the formation of definite though unstable compounds of salts, acids, and bases with water, have been rendered evident by so many careful investigations of experienced chemists,<ref>We need only mention the names of Armstrong, Etard, Pickering, Hendelfieff and so on.</ref> that the secondary importance given to them by most adherents of the physical theory would be simply incomprehensible were it not for the hope which they cherish of ultimately explaining all chemical processes by the above-mentioned molecular movements. At any rate, in order to account for the effects of the chemical reactions, the followers of the physical theory were compelled to seek support in an additional agency—electricity. Starting from the familiar fact of solutions being decomposed by an electrical current, they admitted that in every solution part of its molecules dissociate, breaking up into their component parts, which are charged with either positive or negative electricity (the name of' ions' is usually given to those component parts). By means of this admission, they attempted to explain the discrepancies between observation and the conclusions drawn from the above-mentioned laws, especially in the case of water solutions of salts, acids, and bases, and the stronger solutions altogether. It must be recognised that many important relations between electrical conductivity and chemical action have been brought out in this way by Arrhenius<ref>Sreittka Veteniliapt Aeademient Handlingar, 1863.</ref> and his followers, and many discrepancies between the laws of Van 't Hoff and Eaoult and the observed facts have been explained. But it is also evident that, once a partial dissociation of molecules is admitted, the whole takes a chemical aspect, and reference to such an unknown cause as electricity does not simplify the matter. All kinds of chemical reactions take place in solutions. Some molecules of the dissolved body simply exchange their atoms in succession, while maintaining the same grouping of atoms, and consequently the same chemical composition. In other molecules the grouping only of the same atoms is changed, and we have reactions of replacement, or isomerism. But, at the same time, new and more or less stable combinations between the atoms of both solvent and dissolved body take place in various proportions; double decompositions most probably occur as well; while the physical phenomena of sliding of undecomposed particles continue at the same time—the physical movements of the particles being impressed by, and acting upon, the chemical movements of the atoms within the molecules.
It must be confessed that neither theory has as yet succeeded in following this multitude of movements and of catching the moment when the movements of particles are transformed into atomic movements and re-distribution; and though we may name several equally important works which have been published on this subject during the last twelve months, we can mention none which have thrown new light on the subject.<ref>Besides the leading chemical periodicals, an excellent analysis, by W. Nernst, of all the chief work done during the year 1891, and its bearing upon the theory of solutions, will be found in a chemical yearbook which was started this year by Hichard Meyer, the Jahrbuch der Chemie. Frankfurt, 1892.</ref> Let us only add that the subject itself has been immensely widened of late by the wonderful researches of Heycock and Neville on the lowering of the temperature of solidification of metals, by the addition of other metals, and of Roberts-Austen upon alloys—that is, metals dissolved in metals— which behave very much like all aqueous solutions. However, a new departure in this branch has been made, quite recently, by Messrs. Harold Picton and S. E. Linder. They studied the structure of solutions of sulphide salts which offer the advantage of giving a whole series of gradations between real solutions (that is, liquids which seem to consist of liquid particles only) and such as contain extremely small particles of solid matter in suspension. By submitting the series to various tests, it was ascertained that all these solutions, even those reputed as homogeneous, contain infinitely small solid particles, the presence of which is revealed, on Tyndall's method, by a beam of light. In some of them the particles—all of the same size and performing rapid oscillatory movements—are even seen under the microscope, when magnified a thousand times; while in antimonium sulphide the very formation of coarser agglomerations out of invisible particles can be followed under the microscope. In short, the authors came to the conclusion that there is no sharp limit between a state under which the mutual attractions between the particles of the solvent and the suspended particles of the dissolved body are very feeble, and a state when, these aggregations becoming of a smaller size, the forces which keep them in the solution become of a decidedly chemical nature. A new and promising method is thus given.
If we take into account the rapid accumulation of data relative to the subject of solutions and the various theories already germinating, we may hope that the day is not far off when a complete theory of these phenomena will be possible. Let us only remark that all the work hitherto done confirms more and more the idea which becomes more and more popular among chemists, and which Mendeleeff has so well expressed in a lecture delivered before the Eoyal Institution in May 1889;<ref>An Attempt to apply to Chemistry one of the Principles of Newton's Natural Philosophy,' in the Principles of Chemistry, vol. ii. Appendix L</ref> namely, that the molecules of all bodies, simple or compound, borrow their individualities from the characters of the movements which the atoms perform within the molecules. Each molecule may be considered as a system, like the systems of Saturn or Jupiter with their satellites—each separate type of such systems giving a separate type of molecules, and the chemical properties of the molecules being determined by the character of the system and its movements. It may already be foreseen that further progress in the great investigation into the mechanical basis of chemical energy will be made in this direction.
One of the chief objections to the theory of evolution which was especially laid stress upon some thirty years ago, was the impossibility of producing at that time a series of ' intermediate links' to connect the now existing animals and plants with their presumed ancestors from former geological epochs. To meet the objection, Darwin had to devote a special chapter in his great work to the imperfection of the geological record, and to insist both upon its fragmentary character and our imperfect knowledge of what it contains. The recent progress of both geology and palaeontology renders such explanations almost superfluous. Geology, aided by the deep-sea explorations, has come to a better comprehension of the mechanism of sediments, and it knows what it may expect to find in the rocky archives of the earth, and what it may not; and, on the other side, the discovery of the missing links between past and present has been going on of late with such a rapidity as has outstripped the most sanguine expectations. Our museums already contain whole series of fossil organisms which almost step by step illustrate the slow evolution of large divisions of both animals and plants; our present mammals already have been connected by intermediary forms with many of their Tertiary ancestors; and the palaeontologist can already trace the pedigree of birds, and even mammals, as far back as the lizards of the Secondary period—not merely deducing it from embryological data, but by showing the real beings which once breathed and moved about upon earth.
At the same time one point of great moment for the theory of evolution, and only alluded to by Darwin, has been brought into prominence. The part played by migrations in the appearance of new species has been rendered quite obvious. Thus we know perfectly well that the ancestors of our horse migrated over both Americas, Asia, Europe, Africa, and probably back to Asia, and that each step in those migrations was marked by the apparition of some new characters which are now distinctive of the horse. The same remark applies to the mastodons and their descendants, the elephants; to the common ancestors of the camel and the llama, and to the Ungulate altogether. It may be taken now as a general rule that the evolution of new species chiefly took place when the old ones were compelled to migrate to new abodes, and to stay there for a time in new conditions of climate and general surroundings. The intermediate forms have not been exterminated on the spot; and if we want to obtain the intermediate links between two allied species, the relics of which are found in two geological formations of a given country, we must ransack for fossils all the five continents upon which the intermediate links have been scattered. This is why the discovery of intermediate types has gone on so rapidly since North America, South Africa, South America, New Zealand, and partly Asia began to be thoroughly explored by experienced palaeontologists.
Many of the 'missing links' were discovered, as is known, in Darwin's lifetime. Thus, the first really bird-like, feathered lizard, the Archceopterix, was unearthed as early as 1862; and eight years later, Professor 0. C. Marsh already described, from the Upper Cretaceous beds of North America, two more lizard-birds, one of which (Hesperornis) must have resembled our present fish-eating divers, while the other (Ichthyornis), provided with powerful wings, had —apart from its teethed jaws—all the appearance of a bird of our own time.<ref>B. Lydekker's Catalogue of Fouil Sirdt of the British Museum, London, 1892. For the general reader we cannot bnt highly recommend a charming book of the same author, Phaiei of Animal Life, Pait and Pretent, London, 1892, which is a real aodel of scientific and popular literature.</ref> And, finally, the discovery of a large ostrich-like bird (Dasornis Londinenais) in the Lower Eocene of the Isle of Sheppey, and of another, also big and flightless bird (Oastornis), in the Eocene of Meudon, Kheims and Croydon, established a further connection between the bird-like lizards of the Triassic times and real specialised birds.
These last discoveries brought the series very near to our own times, and they were the more valuable as the just mentioned Gastornis proved to combine some of the characters of both flying birds and of those which, like the ostrich, the cassowary, and the emu, do not fly; while the Pliocene deposits of North India and the numberless remains of the so-called moos of New Zealand yielded specimens of still nearer ancestors of our flightless birds. The New Zealand deposits of bones became known more than fifty years ago, when Owen, on receiving (in 1839) a broken but characteristic moabone, determined the general characters of the great ostrich-like Dinornis, which inhabited the island quite recently, but is found no more in a living state. But it is especially of late that the enormous accumulations of moa-remains have been explored in detail. Cartloads of those bones have already been shipped to Europe, and new accumulations continue to be found—always with the same astonishing numbers of individuals entombed on the same spot, and in the same excellent state of preservation. Such a deposit—one of the most remarkable of its kind—has been lately discovered by Professor H. 0. Forbes, near Oamaru, in the South Island of New Zealand. In a small hollow which did not exceed twelve yards in width, no less than 800 to 900 individuals were embedded in solid peat, under a superficial layer of a few inches of soil. Many skeletons lay quite undisturbed, and in some instances the contents of the stomach, which consisted of triturated grass and small rounded and smoothed quartz pebbles, were found lying in their natural position, under the sternum. The bones of a giant buzzard, a big extinct goose, the Cape Barren goose, the kiwi, and so on, were mixed together with bones and full skeletons of several species of Dinornis, big and small.<ref>Letter to Nature, March 3,1892, vol. xlv. p. 416.</ref> And again, as on previous occasions, the New Zealand scientists are at a loss to explain the accumulation of so many various birds on such a narrow space. However, the most interesting part of Professor Forbes's discoveries is that he has finally succeeded in finding among this mass of bones one bone, at least, which bears unmistakable traces of having been connected with a humerus, the head of which must have been as substantial as in cassowaries. He thus considers it proved that the Dinornithidse, like the kiwis, descended from birds which could fly.<ref>Ibid. 1892, vol. xlv. p. 257.</ref> The last missing link is thus discovered, and the chief points in the genealogy of birds are thus already settled, while many a gap which still remains will certainly be filled up when the rich materials recently excavated in both Americas have been carefully examined by anatomists.
The same may also be said in regard to mammals, if the recent discoveries in North and South America are taken into account. The earliest traces of mammals have been found, as is known, in the Triassic deposits of Germany, Basutoland, the Cape Colony, and North Carolina; and it is also known, through the previous remarkable works of Professors 0. C. Marsh and H. F. Osborn, that the Jurassic deposits of Wyoming have yielded a rich fauna, among which we find the remote ancestors of various orders of the present mammalia.<ref>0. C. Marsh, in American Journal of Science, 1888 to 1891; H. F. Osborn, 'The Structure and Classification of Mesozoic Mammalia,' in Journal of the Academy of Natural Science of Philadelphia, vol. iz.; R. Lydekker, Catalogue of the Foiiil Mammaliai in the Sritith Jfvseum, London, 1891.</ref> But the most important finds which throw a new light both on the earlier and the subsequent forms, have been made in that immense area of lacustrine beds which have been deposited in the region of the great salt lakes of Utah, Wyoming, and Colorado, from the end of the Cretaceous period down to the middle parts of the Tertiary epoch. There, and especially in the Eocene ' Puerco' and 'Wasatch' beds, as well as in the Eocene 'Uinta' formation, a rich fauna of mammals has been unearthed.<ref>Cope's Synoptis of the Vertebrate Fauna of the Puerco Seriet, and W. Scotland H. F. Osborn, ' The Mammalia of the Uinta Formation,' in Trantactiom of the American Philosophical Society, new series, vol. rvi. Parts II. and III. Philadelphia, 1889. Also B. Lydekker's paper in Nature, vol. xliii. p. 177; and Pha*et of Animal Life. </ref> All of those Eocene mammals had something in common in their leading features, and yet they offered a sufficient diversity for being considered as the probable ancestors of nearly all orders of placentar mammals. To mention their feet only, they were adapted, in all of them, for walking upon the sole, and were provided with five toes; but it is easy to recognise in the structure of the feet of the different genuses such divergences as necessarily ought to evolve, under certain conditions —on the one side, the plantigrade foot of the bears, and, on the other side, the digitigrade foot of the Ungulata (horses, camels, elephants, and so on), who walk upon the points of their toes; and, again, among these latter it is possible to find indications for an evolution which must have ended in the appearance of two divisions —the odd-toed and the even-toed ungulates. Most laborious anatomical researches were required for properly interpreting these rich, materials. But the result of the work is that we already know with a great approach to certitude the genealogical-trees of most ungulates; we can go back to the ancestors of the ruminants, the cameloides, the chevrotains, the horses, and even to the common ancestors of the whole group of ungulates; while the genealogy of other large groups of mammalia has also been worked out to some extent.
The just-mentioned discoveries in North America were soon supplemented by still more remarkable finds in South America, which finds follow each other with such a rapidity that anatomists will have to make strenuous efforts in order to keep pace with the palaeontological work. The formation which D'Orbigny described as 'formation guaranienne' proved to consist of marine Cretaceous beds, covered by immense land deposits, which, like the Lararnie beds of North America, are of an intermediate age between Cretaceous and Tertiary. These last beds offer an immense interest, owing to their mammalian fossils (of much more specified types than those of the Laramie), which are mixed together with relics of gigantic Dinosaurians, some of the latter attaining lengths of more than 130 feet. As to the more recent deposits of the Argentine Republic and Patagonia—partly Eocene and partly Pliocene—they are so rich in mammals that more than two hundred species, some of them of the most extraordinary types, have already been described by Dr. F. Ameghino,<ref>His chief works are: Lot mamiferot fttilet de la Ameriaa del Snd, Bnenos Ayres, 1880; Contribution al conocimiento de lot mamiferot fosilet de la Itejivblica Argentina., 2 parts, forming vol. vi. of Actas de la Academia de Cieneiat de Cordoba, Buenos Ayres, 1889; and several papers in Revitta Argentina de Hittoria Natural, Buenos Ayres, 1891.</ref> Burmeister, and Moreno; and every number of the Revista Argentina brings some new descriptions of new fossils both from the Argentine and Patagonia, which is now explored by Carl Ameghino. There are among them ungulates which, to use Mr. Lydekker's words, are ' totally unlike any found in all the rest of the world put together,'<ref>Nature, vol. xlv. p. 608.</ref> and which combine the characters of both the odd-toed and the even-toed ungulates. Of them, the Macrauchenia seems to be a direct descendant of a type which must have been a common ancestor to both divisions. Another huge mammal, one of the Toxodontes, which must have equalled in size the hippopotamus, also occupied an intermediate position between the two groups; while in the earlier Tertiaries there are types which, so far as can be judged from preliminary descriptions, must have stood near the source from which both ungulates and rodents have taken their origin. Very many interesting Edentata and rodents have been met with in the same beds, but it is the marsupial group which surpasses all others in interest. One carnivorous animal of this group (Prothylacinus) is almost identical with the now existing pouched wolf (Thylacine) of Tasmania; while another fossil genus (Protoproviverra) is quite akin to the most characteristic carnivorous marsupial, the Tasmanian Devil. Although F. Ameghino's descriptions are not yet complete, the best authorities on this subject in this country and Germany do not hesitate to recognise a purely Australian type in these South American forms, which, on the other side, can safely be connected with the group of primitive earnivors (Hycenodon, Pterodon, &c.) which appeared at a later epoch in Europe. Moreover, the same beds contain fossil remains of primates (Homunaulus, Anthropops, Homocentrus, Eudiastatus) which seem to represent ancestors of all the subsequent apes, but stand also in connection with the lemurs, and also with the ungulates, or, rather, with their Toxodon ancestors. They seem to represent the most ancient primates known, and indicate that the first representatives of the whole group must be sought for as far back as the end of the Secondary period. Finally, we must mention the discovery of remains of man which are considered by F. Ameghino as belonging to the Pliocene and Miocene ages.<ref>The Revista Argentina contains in its issue for December last a full description of the primates discovered by Carl Ameghino in South Patagonia. The connections which these fossils indicate between man, primates, ungulates, and rodents, are of the highest interest.</ref>
The ' missing links' are coming, as we see, in such abundance that it will take several years before anatomists, in whose hands this rich material will now be put, have disentangled the numerous and striking affinities between so many different types which we have briefly enumerated. But geologists will also have a word to say about these discoveries, which raise again the very great question as to the long since noticed affinities between the faunas of all southern continents, and the presumed previous connection between those continents. Apart from all other considerations, the resemblance between the fossil marsupials of South America and the marsupials now living in Australia is so great that it is not possible to admit that forms so near to each other (and both so abnormal) might have developed independently upon two remote continents. It seems almost unavoidable to admit that some direct land connection has existed between South America and Australia, although all we know about the persistence of the chief outlines of the continents seems to be opposed to the admission. Dr. Ihering, who has devoted a good deal of time to the study of the fauna of South America, boldly concludes from his own special researches that during the Secondary period a great continent extended from Chili and Patagonia, through New Zealand, to Australia, while the connection between South America and North America was broken during both the Cretaceous period and a great part of the Tertiary epoch. The striking differences between the faunas of both Americas, and the identity of many representatives of the faunas of South America and South Africa, make him also conclude that the two latter continents were connected as late as the Oligocene period.<ref>Revista Argentina de Cieneia Natural, No. 4 (' Sobre la distribucion geograflca de los Creodontes, and letter to F. Ameghino).</ref> E. Lydekker, whose opinion has such a weight in the matter, also concludes from the many known affinities between the fossil faunas and floras of the four great southern prolongations of the continental mass of the globe that they must have stood in a more or less intimate connection, and have been partially isolated from the more northern lands.<ref>Nature, 1892, vol. xlvi. p. 12.</ref> As to F. Ameghino, he also recognises that, at least during the Oligocene times, South America was in direct connection with the Old World; but he points out the similarity of the mammalian and Dinosaurian faunas of both Americas, and concludes that the two continents must have been connected together, as well as North America with Europe, at an earlier epoch.
It would be premature to attempt now the solution of this complicated question. It may be permitted, however, to point out that the hypothesis of a submerged Antarctic continent is not improbable from the point of view of the physical geographer. The permanence of the continents, which is a fact, and seems to be opposed to the hypothesis, must be understood in a limited sense. In the equatorial and the two temperate zones we undoubtedly have huge continental masses, the great plateaux of Asia, both Americas, and Africa, which, so far as our knowledge goes, have not been submerged since the primary epoch; and around these back-bones of the continents we have huge masses of land which have not been under the sea since the end of the Secondary period. But their outskirts have witnessed several retreats and invasions of the ocean, or of its Secondary period seas. Moreover, the permanence of the continents does not seem to extend to the circumpolar zones. When we consider the outlines of the two great plateaux of East Asia and North America, we see that these two great continents of the Secondary epoch were narrowing at that time towards the north, and that their extremities were pointing towards some spot in the vicinity of what is now the Behring Strait, in the same way as South America, South Africa, and South Australia are now pointing towards the South Pole. The great plateau of Northeast Asia, which has remained a continent ever since the Devonian age, has so much the shape of a South America pointing north-east that the resemblance is simply striking.<ref>Petermann's and Habenicht's map of Asia, in Stieler's Hand-Atlat (No. 58), shows this shape of the plateau better than any other map. For more details see my map in the ' Orography of East Siberia,' in the Memoirt of the Rutfian Geographical Society, 1875, vol. v. (Russian).</ref> On the other side, we know that the Miocene flora discovered in Greenland, Spitzbergen, and New Siberia indicates the existence of a great Miocene continent where we now have but the ice-clad arctic archipelagos. So that we must conclude that, while the central (temperate and equatorial) parts of the globe really offer a certain permanence in the disposition and general outlines of their continents, the Arctic region stands in a different position. It was under the ocean during a large part of the Secondary period, it emerged from the ocean and was occupied by a large continental mass during the Tertiary period; and now it is again under water. Such being the conditions of the Arctic region, we may suppose that the same oscillations took place in the Antarctic region as well. In such case, the two circumpolar regions would have been periodically invaded by the ocean (either alternately or during geological epochs closely following each other), and they would have periodically emerged from the sea in the shape of continents more or less indented by gulfs and channels. In short, a certain stability in the distribution of land and water in the equatorial and temperate zones, and unstability in the circumpolar regions (with, most probably, an unstable Mediterranean belt), would perhaps better express the observed facts than a simple affirmation of stability of continents. If these considerations prove to be correct—and I venture to express them only as a suggestion for ulterior discussion—then the hypothesis of a former more or less close landconnection between the southern extremities of our present continents would not appear unlikely, and the striking similarity between the faunas of Patagonia and Australia would be easily accounted for.
Few branches of science have developed with the same rapidity as bacteriology during the last few years. The idea that infectious diseases are due to some micro-organisms invading the body of the infected animal is certainly old. It was ventilated many hundreds of years ago; and it was revived early in our century. But scientific bacteriology is of quite recent creation. It dates from the end of the fifties—that is, from Pasteur's researches into the fermentation of beer and wine and Virchow's investigations into cellular pathology. Progress has been very rapid since. We have now numerous works, large and small, devoted entirely to the description and study of the life-history of the microscopic organisms which occasion disease; and every year brings the discovery of some new micro-organism to which some disease, or group of diseases, may be attributed. Cholera, typhoid fever, gastric affections altogether, malaria, and influenza; tuberculosis, leprosy and cancer; diphtheria, measles, and scarlet fever; rheumatism, anthrax, small-pox, rabies and tetanus; nay, even the poison of the cobra snake,<ref>M. Calmette, in Archive! de mtdecine navale et coloniale, Mars 1892; referred to in Jierue Scientifiyue, 23 Avril, 1892.</ref> have been traced to separate microscopical beings. The photograph of each separate bacillus or micrococcus may be found in the text-books; its manners of life, and very often its modes of reproduction, have been carefully studied, both in the animal body and in artificial cultures; so also its morbid effects when introduced into the bodies of various animals. True that the general reader is often amazed on learning that such and such a microbe which was introduced a few months ago, as the real cause of influenza or of some other disease, is recognised now as a common inhabitant of the human body, and has nothing to do with the said disease; while a few months later the real enemy will again be discovered, but will have no more success than its predecessor. But such ephemeral discoveries are simply indicative of an unhappily general tendency among modern scientists —that of hastening to announce discoveries, and to attach one's name to something new, before the supposed discovery has been submitted to the test of searching experiment. The same tendency prevails in all sciences—the only difference being, that the general reader is seldom gratified by the daily press with the discovery of a new chemical 'law,' or of a new 'type' of fossil mammals, while each discovery which deals with disease, ephemeral or not, enjoys a wide publicity so soon as it has found its way into a scientific periodical. The very rapidity with which the would-be discoveries of new bacilli are reduced to their real value only proves, on the contrary, the safety of the methods used by bacteriology for distinguishing between the seeming and the real causes of diseases.
We may thus safely recognise that science already knows a great number of micro-organisms which are capable, under certain circumstances, of producing certain specific diseases; and we may note that even those researches which, at the first sight, seem to overthrow established facts, only result in a deeper knowledge of diseases and their modifications. Thus, the recent investigations of MM. Lesage and Macaigne, who have finally succeeded in differentiating the typhoidic bacillus from the Bacterium coli—a microbe which is constantly met with in our intestines, and only under certain conditions acquires an especial virulence—are one of the best examples of how further research deepens our knowledge of microbes; and Dr. Cunningham's discovery of ten different varieties of the choleraic bacillusn certainly will have the same effect: it will simply widen our knowledge of the different forms assumed by cholera.
Things stand, however, quite differently with the means of combating infectious micro-organisms. Most of the specifics which once awakened so many hopes have proved in the long run to be as ineffective against bacilli as the specifics periodically proposed by allopaths and homoeopaths are powerless against the diseases themselves. And the more the study of bacteria is advancing, the more it is recognised that a healthy body which is capable of itself putting a check on the development of morbid micro-organisms is the best means of combating them; that sanitary measures which prevent the very appearance of morbid germs are the surest means against the possibilities and the risks of infection. But what permits a healthy body to resist its invasion by morbid organisms? What gives several animals immunity against certain special diseases? Why do rats resist anthrax, and dogs and monkeys resist the tuberculosis of fowls, while the same microbes are fatal to rabbits and guinea-pigs? And how can immunity against certain diseases be acquired, either cauterised spot behind. A decoction of dead leaves attracts it, while a solution of sugar or salt repels it.<ref>Scientific Memoirs by the Medical Officers of the Army of India, Part VI.; analysed in Annaleide MicrographTM, 1892.</ref> The same is known of isolated amcebse. So also the leucocytes immediately attack and ingest some microbes, living or dead, but avoid some others, and various kinds of leucocytes behave in various ways. The mono-nuclear leucocytes of man seem loth to attack the bacDli of erysipelas, while the manynuclear ones display no such reluctance. Altogether, some substances exercise upon leucocytes a decidedly attractive power, while other substances repulse them.
As to what happens with microbes which have been ingested by leucocytes, the result may be very different in various conditions. The red corpuscles of blood, when ingested by leucocytes, are digested; globules of pus and fragments of muscular tissue also are digested by means of a special ferment (discovered in 1890 by Rosbach). And the rsame happens with microbes if the leucocytes of the organism are healthy and the animal is refractory to a given disease, either from natural causes or in consequence of vaccination. The bacilli of anthrax are undoubtedly destroyed by the leucocytes of the dog, as well as by those of such rabbits as have been vaccinated against anthrax. If the leucocytes are healthy, they prevent the germination of the spores which they have ingested; but they maintain this power so long only as they are healthy; because, if the animal has been submitted to cold (or to heat in the case of a frog), or if it has been narcotised,x it loses its immunity. Moreover, the very affluence of phagocytes to an infected place may be accelerated through nervous action, or slackened by various narcotics.
Such being the facts, it was quite natural to explain them, as Metchnikoff did, by maintaining that the phagocytes are the natural means of defence of organisms against infectious disease. The very necessities of struggle for life have evolved this capacity of the organisms of protecting themselves by sending armies of phagocytes to the spots attacked by noxious micro-organisms. The struggle may evidently end in either the defeat of the phagocytes, in which case disease follows, or the defeat of the microbes, which is followed by recovery; or, the result may be an intermediate state of no decisive victory on each side, as is the case in various chronic diseases.H As to the force which attracts the leucocytes towards the microbes,