LINKS IN THE CHAIN.

CHAPTER I.

AN UNSEEN WORLD.

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Where the pool

Stands mantled o'er with green, invisible,

Among the floating verdure millions stray."

Ir Dr. Livingstone, in the course of his African explorations, were to light upon some such nation of mannikins as that discovered at Lilliput by Mr. Lemuel Gulliver of famous memory, the announcement of the fact could hardly occasion greater surprise and astonishment than was felt by the philosophers of the last century at the remarkable disclosures of the microscope, with respect to the countless multitudes of minute forms of organic life which, unseen by our feeble powers of vision, people the waters of the earth, and swarm around us on every side.

The microscope has here revealed the existence of a new world of living beings, before unknown and unsuspected. It has shown that the water of our roadside ditches, and stagnant pools, of our lakes and streams, and of the sea itself, teems with various races of microscopic beings, many of which are so inconceivably minute, and abound in such amazing profusion, that thousands of them may often be detected in a drop of fluid taken up on the point of a pencil, while, within the narrow dimensions of a lady's thimble, we might easily collect together a number far exceeding that of the entire human population of the globe.

It rarely happens that important discoveries are made at once in all their completeness, and the discovery of this unseen world of organic life forms no exception to the rule. It will be obvious, indeed, that, as many of these minute beings are sufficiently large

B

to be discernible by the unassisted eye, there would be at least some knowledge of their existence even in the earliest times. But it was not until the invention of the microscope had given to man the means of penetrating the mysterious depths of this diminutive creation that any definite knowledge was obtained, either of its vast extent or of the exceeding wonderfulness of the beings which composed it; while almost every improvement which has subsequently been made in the construction of the microscope has largely added to the wonders which the instrument has here revealed.

The earliest microscopical observations of importance on these minute beings were made by Leeuwenhoek in the latter half of the seventeenth century; but it was not until a much later period that this branch of study excited any great amount of interest, or was prosecuted with anything like persevering zeal. It was towards the middle of the last century that the great stimulus was given to researches of the kind, by the publication of the celebrated memoir of M. Trembley, of Geneva, on the little Hydra, or fresh-water polyp of our ponds and ditches-an animal whose extraordinary history read, and, indeed, still reads, more like an extravagant romance than a sober recital of actual facts. The publication of this work marks one of the most important epochs in the history of microscopical inquiry, and speedily turned the attention of observers in all parts of Europe to the abounding marvels of the waters.

Nor is it at all remarkable that such should have been the result; for this same Hydra is one of a race of animals of which, up to that time, naturalists had been almost wholly ignorant, while the details that were given of its habits and economy were such as completely upset most of the prevailing notions as to the distinctive characteristics of animal life. For here was an animal whose young grew out from its sides like buds from the stem of a plant—that might not only be cut into pieces without being killed, but would reproduce a perfect animal similar to itself from every fragment of its original substance that might be turned inside out, and yet suffer no detriment to its powers of digestionthat might, in fact, be mutilated and operated upon in almost any conceivable fashion, not only without apparent injury, but without, or at all seriously, interfering with the functions of active life. It is no wonder, therefore, that observers were everywhere

INFUSORIAL ANIMALCULES.

3

eager to test the accuracy of these astounding assertions, or that a great impetus should thus have been given to the general study of the minute forms of life with which the waters abound. But many years still passed before this branch of study was at all systematically pursued: and although towards the close of the last, and early in the present, century, it attracted to itself a considerable number of able and zealous investigators, it was not until almost our own day that it took its place as a recognized and important department of natural science.

It was for many years customary to speak of these living atoms indiscriminately as the Infusoria, or as Infusorial Animalcules, from the circumstance that they were originally detected in infusions of vegetable substances. More careful investigation, however, has shown that the organisms thus hastily associated together differ widely from each other; and that many of the tribes are, on the whole, more nearly related to the vegetable than to the animal world. Moreover, amongst those whose animal nature is perfectly obvious, the differences which have been observed as to organization are so great, that while most of the number take rank with the very humblest forms of sentient beings, others, again, present a much more complicated structure, and are now properly classed with the higl.er divisions of the invertebrate animals, minute though they be, and, in this respect, true animalcules. It would seem, indeed, that the earlier investigators of this invisible world of organic life were so elated with the remarkable discovery they had in charge, that they were disposed to regard everything belonging to it as perfectly unique, quite separate and distinct from all that had previously been known. No matter, therefore, what the objects were that came within the field of their microscopes - the eggs and larvæ of insects, the germs of embryo polyps, the spores of fungi, and many different forms of mature plants, together with other matters of a still more heterogeneous character—all were alike set down as being organisms of the same general character, and regarded as equally belonging to this newly discovered world of Infusorial Animalcules,

It affords a curious illustration of the union of great powers and special aptitude for delicate research with unaccountable proneness to deception, that Ehrenberg, the great microscopist of Berlin, and for many years the almost undisputed authority on

4

EHRENBERG'S MISTAKES.

the Infusoria, was beguiled into more errors of the kind above alluded to than almost any other observer. In his great work, "Die Infusionsthierchen," published in 1838, and which, with all its imperfections, is undoubtedly a splendid monument of his industry and genius, there are perhaps a greater number of assumed discoveries which no subsequent observer has ever been able to confirm, and a greater number of inferences which are now discarded, although deduced from acknowledged phenomena, than are to be found in any similar work in the whole compass of modern science. And yet, when first published, this work seemed so complete and so exhaustive of the subject, that it was some years before any one doubted either the correctness of the observations or the soundness of the conclusions of its author. The name of Ehrenberg was a sufficient guarantee for any statement, however marvellous, and his reputed discoveries passed for a time unquestioned. Gradually, however, as the number of observers increased, and as various improvements were made in the construction of the microscope, the views of the great Berlin philosopher were discovered to be in many particulars erroneous, and that one of the two great characters on which he had relied for separating the Infusoria into their main divisions, was altogether imaginary.

According to the classification adopted by Ehrenberg, the Infusoria formed two principal and well-defined groups. To the first of these he gave the name Polygastrica, from the supposition that all the animalcules composing it possessed several distinct stomachs, united together by a sort of intestinal canal. The second group consisted of the Wheel Animalcules, so named on account of the wheel-like rotation produced by rows of delicate hair-like bodies, resembling eyelashes, and hence termed cilia, which fringe the upper extremity of the bodies of the animalcules; and to these Ehrenberg gave the name Rotatoria, or Rotifera. In these two groups, as the primary divisions, all the Infusoria were included; and though Ehrenberg was fully sensible of the great dissimilarity between the organization of the Rotifera and that of the various forms included in the other group, he seems to have had no doubt as to the perfect propriety of their being all retained under one and the same general designation. Never, however, have the views of a philosopher, or his supposed discoveries, been more completely set aside by subsequent observa