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the persevering zeal which they devoted to the analysis of higher forms of animal and vegetable structure. Its wonders, however, were gradually unfolded; so that in the various treatises on the microscope, published during the eighteenth century, they occupy a conspicuous place. In the meantime, the microscope had serious difficulties to contend with, some from the nature of the case, some accidental. The defects inseparable from its original construction formed a bar to all discovery beyond certain limits. But it fell under a temporary cloud from another cause. When physiologists began to theorise on the elementary structure of the animal body, and to twist their imperfect observations into accordance with their theories, there soon followed a general suspicion of a want of trustworthiness in the microscope, and in everything announced upon its authority. The instrument and its advocates were brought into more or less discredit, the effect of which has continued to attach to them, in the minds of many, until the present day. We wish, therefore, to pause for a moment, and examine into the grounds of this impression.
A tendency, common to all observers, is to describe what they believe and infer, rather than what they actually see. Microscopic observers were especially open to this reproach, as long as a want of definiteness in the image presented to their eyes, left so much to be completed by the imagination. But from the moment that the visual image, presented by a well-constructed microscope, gave as good an idea of the object as we could have obtained from the object itself, if enlarged to the same size, and viewed by the unassisted eye, microscopic observations were made with the same degree of certainty in this respect, as observations of any other class. Another fallacy, common indeed to all observers, but of which the microscopic observations of former times had perhaps more than their share, arose from not sufficiently attending to a difference in the circumstances under which the observations had been made. Thus one observer described the human blood-corpuscles as flattened discs, like pieces of money, another as slightly concave on each surface, another as slightly convex, and another as highly convex, or even globular. Yet all microscopists now agree that their real form, when examined in freshly-drawn blood, is that of
discs with slightly-concave surfaces. The diversity in the previous accounts was simply due to the admixture of water or other liquids added for the sake of dilution, which, being absorbed by the blood-discs, at first rendered their surface flat, then convex, and at last changed their form into the globular. But microscopical inquiries are no longer more exposed to fallacies of this description than any other branch of physiology. And the microscopists of the present day can be only held responsible for the errors of their predecessors, by persons who are ignorant of the perfection which the instrument has now attained, as well as of the general agreement regarding facts among all who are competent to use it, however they may differ in their inferences.
But our immediate object is to introduce our readers to some of the most interesting discoveries for which we are indebted to the recent improvements in the microscope: and we propose to pass lightly over such as the public may be presumed to be acquainted with, from their having been incorporated into treatises adapted for general circulation. We shall say little, therefore, of Professor Ehrenberg's laborious investigations into animalcular life, as well fossil as recent; although his additions of new forms of organized beings to the catalogue of the naturalist greatly exceed all that have been contributed by the most industrious collector in any other department. It may give some idea of their wonderful variety, to be told that Professor Ehrenberg detected in some specimens of a tertiary deposit, brought a short time since from the island of Barbadoes by Sir R. Schomburgk, no fewer than 1,200 new specific forms. It was necessary to create 250 new genera for their reception: the mere naming of which must have constituted in itself no insignificant labour. Professor Ehrenberg first discovered, that various extensive deposits, both calcareous and silicious, are almost entirely made up of the remains of minute organisms; incalculable numbers of which must have successively lived and died, flourished and decayed, leaving the solid portion of their fabrics to accumulate during countless generations, and thus to form vast beds of chalk or strata of silicious sand. One of the most interesting discoveries of this class was recently communicated to the scientific world by Mr. W. C. Williamson.
By means of a microscopic analysis of the white mud, with which the sea-bottom of a great part of the Levant is covered, he has shown that the deposit consists almost entirely of organic forms: and among them may be distinguished the shells of about twenty-five distinct species of animalcules, some silicious, others calcareous; besides certain other bodies hereafter to be noticed, which are determined by the microscope to be fragmentary particles of shell.
We owe to Professor Ehrenberg the separation of the remarkable group of Rotifera or Wheel Animalcules, from a lower division,-to which he has given the name of Polygastrica. On the complex structure and high organization of the Rotifera, all microscopists are agreed; not so, with respect to the Polygastrica. Many distinguished naturalists question the high authority of Professor Ehrenberg in this instance: and maintain that the facts which he has observed do not bear out his inferences, and that his account of the digestive apparatus of the Polygastrica is not entitled to rank as a physiological truth. Into this question we shall not now enter. But it is right to state our conviction, that Professor Ehrenberg has included among his Animalcules many forms which are truly vegetable, and whose supposed polygastric structure is altogether a product of his imagination. The beautiful tribe of Desmidea, which forms the subject of Mr. Ralfs's admirable monograph, is now almost universally admitted, in this country at least, to be of a vegetable nature: and the recent discovery by Mr. G. H. K. Thwaites of the conjugation of the Diatomeæ makes it next to certain that this group, to which a very large proportion of the silicious fossil animalcules (so called) belongs, must be transferred in the same manner to the domain of the botanist. The phenomenon in question is one of a very singular character: and, as it is one of which the right understanding will probably lead to a remodelling of the whole fabric of Cryptogamic Botany, we shall endeavour shortly to explain it.
The attention recently paid to the simplest forms of vegetable existence has shown, what was previously suspected, that in the simple cell of the lowest Algae we may find the type of all vegetative existence. This cell grows
from a germ, draws together the nutrient materials supplied by the surrounding air and moisture, combines them into new products, appropriates these products to itself by incorporating them with its own structure, multiplies its kind when it has itself attained maturity, and finally dies after performing all the functions of the most highly organized plants. And yet this cell is but a solitary sacculus, or minute bag, whose wall is composed of a transparent colourless membrane, whilst its cavity contains a fluid, in which are held a number of granular particles of a red or green hue, mingled with others that consist of starch. Of aggregations of such cells, each of them a distinct individual, the 'red snow,' the 'gory dew,' the green matter of Priestley,' and various other low forms of vegetation developing themselves in damp situations, are chiefly composed;-the cells being generally united by a gelatinous substance in which they are imbedded-but each one living for and by itself alone. Now, even in these insignificant tribes, we have the two modes of reproduction, as possessed by the higher plants, clearly sketched out. Whilst the cells continually multiply themselves by a system of subdivision, in which each splits as it were into two, an entirely new generation arises from the approximation of two cells, and the mingling of their contents. The immediate result is the formation of a sporangium, from which the new family is to spring, as the higher plant does from its seed. Now the process of multiplication, by the mere doubling of cells, is obviously analogous to the multiplication of the flowering plant by leaf-buds; the same that, in fact, takes place whenever there is a new growth in continuity with the old. But the act of conjugation, and the formation of the sporangium, represent the essence both of the flowering of the higher plants and of the ripening of the ovule. The fertilization of this consists (as we shall hereafter see) in the mixture of the contents of the two cells which are set apart for the purpose, and which differ from the conjugating cells of the lowest Cryptogamia in no particulars of importance.
The process of conjugation has long been known to take place in a certain genus of Confervæ, which received the name of Zygnema in consequence. But the phenomenon was considered exceptional until within a very recent
period. It has lately, however, been recognised among so many other tribes of the lower Algæ, that it must henceforth be looked upon rather as the rule. The fact that the origination of new generations proceeds from conjugation, or from the union of the cell-contents of two parent individuals, as well in the lowest Cryptogamia as in the highest flowering-plant, has obviously a most important physiological bearing-for it leads us to revert to the opinions of those naturalists who have maintained the existence of sexes in the Cryptogamia generally. At the same time it affords a valuable means of discrimination in regard to such ambiguous forms as the Desmidea and Diatomeæ, whose animal or vegetable nature may be probably determined by this test more satisfactorily than by any other; nothing that truly resembles the conjugation of plants having been yet seen in any tribe of an undoubtedly animal character.
The careful study of these simple forms of vegetation has also led to the discovery that active movements, strongly resembling those of many animalcules, are performed by bodies whose vegetable character is undeniably proved by the nature of the structures into which they are subsequently developed; in which fact we have the explanation of the strange doctrine of former microscopists, who taught that there are beings which are animals in the earlier stage of their existence, and plants in the later. These movements depend on the same agency as those of many lower animals, namely, on the vibration of little hair-like filaments, termed cilia, from their resemblance to eyelashes. The cilia first strike the water with a broad surface, like that of an oar; whilst, in returning to their original position, they present a feather-edge to the liquid, and thus propel with great energy the body to which they are attached. This ciliary movement however is not confined to animalcules. By means of it the oyster, though it does not itself move, yet creates a current in the surrounding water-bringing food to its mouth, and oxygen to its gills. In higher animals it covers many of the membranous surfaces, especially those of the respiratory organs. Even in man there are incalculable numbers of such cilia constantly at work in his air-passages, which prevent the accumulation of the mucous secretions in the smaller tubes, and assist in carrying them away. This addition