threshold of that dark region where Animal Will begins; and yet there is here only the simplest form of organization.*

51. Now let our glance pass on to the second stage the Cell. Here we have a recognized differentiation in the appearance of a nucleus amid the protoplasm. The nucleus is chemically different from the substance which surrounds it; and although perhaps exaggerated importance has been attributed to this nucleus, and mysterious powers have been ascribed to it, yet as an essential constituent of the cell it commands attention. Indeed, according to the most recent investigations, the definition of a cell is "a nucleus with surrounding protoplasm." The cell-wall, or delicate investing membrane that which makes the cell a closed sac is no longer to be regarded as a necessary constituent, but only as an accessory.+

* Here organization is the simplest form of all-molecular organized structure, which in the higher forms becomes tissue structure, and organ structure. The word structure properly means orderly arrangement of different materials; and molecular structure refers to the different proximate principles which constitute the organized substance. Usually, however, the word structureless indicates the absence of visible arrangement of the parts; a cell has structure since it has nucleus and protoplasm.

In the cell-theory established by SCHLEIDEN and SCHWANN, in 1838, and which has formed the basis of modern histology, the cell-wall was endowed with an importance which can no longer be upheld now that the existence of independent organisms, and of cells, without a trace of enveloping membrane has been abundantly observed. Cells without walls were first described by COSTE in the Comptes Rendus, 1845, p. 1372. They were also described by CHARLES ROBIN in 1855, Dict. de la Médicine, art. Cellule. But little notice was taken until MAX SCHULTZE, in his famous essay, Ueber Muskelkörperchen und was man eine Zelle zu nennen habe, which appeared in Reichert und Du Bois Reymond's Archiv, 1861, BRUECKE, in his memoir, Die Elementarorganismen, 1861, — and LIONEL BEALE, in his Structure of the Simple Tissues, 1861, - all about the same time began the reform in the cell-theory which has effected a decisive change in the classical teaching. LEYDIG claims, and with justice, to have furnished important data in this direction (Vom Bau des thierischen

52. The cell may be either an organism or an organite. It may lead an isolated life as plant or animal, or it may be united with others and lead a more or less corporate existence; but always, even as an element of a higher organism, it preserves its own individuality. At first we see that the corporate union is very slight, merely the contact of one cell with another of its own kind, as in the filament of a Conferva. Rising higher, we see the cell united with others different from it; plants and animals appear, having structures composed of masses of various cells. Rising still higher, we see animal forms of which the web is woven out of myriads upon myriads of cells, with various cell-products, processes, fibres, tubes.

ORGANISM AND MEDIUM.

53. But we have only one half of the great problem of life, when we have the organism; and it is to this half that the chief researches have been devoted, the other falling into neglect. What is that other? The Medium in which the organism lives. Every individual object, organic or inorganic, is the product of two factors: first, the relation of its constituent molecules to each other; secondly, the relation of its substance to all surrounding objects. Its properties, as an object or an organism, are the results of its constituent molecules, and of its relation to external conditions. Organisms are the results of a peculiar group of forces, exhibiting a peculiar group of

Körpers, 1864, I. p. 11). The student interested in this discussion should consult MAX SCHULTZE, Das Protoplasma der Rhizopoden und der Pflanzenzellen, 1863; HAECKEL, Die Radiolarien, 1862; the controversial papers by REICHERT, in his Archiv (beginning with the Report of 1863), and MAX SCHULTZE, in his Archiv für mikros. Anat., with HENLE's judg ment in his Jahresberichte, and KÖLLIKER'S summing-up in the last edition of his Gewebelehre. For a full yet brief history of the cell-theory see DRYSDALE, The Protoplasmic Theory of Life, 1874, pp. 96–106.

phenomena. Viewing these in the abstract, we may say that there are three regulative laws of life: (1) The Lex Formationis-the so-called nisus formativus, or "organizing force"; (2) the Lex Adaptationis, or adaptive tendency; (3) the Lex Hereditatis, or tendency to reproduce both the original form and its acquired modifications. We have always to consider the organizing force in relation to all surrounding forces a relation succinctly expressed in the word Adaptation. Just as water is water only under a certain relation of its constituent molecules to the temperature and atmospheric pressure-just as it passes into other forms (ice or steam) in adapting itself to other conditions; so, likewise, the organism only preserves its individuality by the adjustment of its forces with the forces which environ it.

54. This relation of Organism and Medium, the most fundamental of biological data, has had a peculiar fortune: never wholly unrecognized, for it obtrudes itself incessantly in the facts of daily experience, it was very late in gaining recognition as a principle of supreme importance; and is even now often so imperfectly apprehended that one school of philosophers indignantly rejects the idea of the Organism and Medium being the two factors of which Life is the product. Not only is there a school of vitalists maintaining the doctrine of Life as an entity independent both of Organism and Medium, and using these as its instruments; but there is also a majority among other biologists, who betray by their arguments that they fail to keep steadily before them the fundamental nature of the relation. Something of this is doubtless due to the imperfect conception they have formed of what constitutes the Medium; instead of recognizing in it the sum of external conditions affecting the organismi. e. the sum of the relations which the organism maintains with external agencies, they re

strict, or enlarge it, so as to misapprehend its significance restrict it to only a few of the conditions, such as climate, soil, temperature, etc., or enlarge it to embrace a vast array of conditions which stand in no directly appreciable relation to the organism. Every one understands that an organism is dependent on proper food, on oxygen, etc., and will perish if these are withheld, or be affected by every variation in such conditions. Every one understands that an animal which can devour or be devoured by another, will flourish or perish according to the presence of its prey or its enemy. But it is often forgotten that among external existences, all those which stand in no appreciable relation to the organism are not properly to be included in its Medium. In consequence of this oversight we frequently hear it urged as an objection to the Evolution Hypothesis, that manifold organisms exist under the same external conditions, and that organisms persist unchanged amid a great variety of conditions. The objection is beside the question. In the general sum of external forces there are certain items. which are nearly related to particular organisms, and constitute their Medium; those items which are so distantly related to these organisms as to cause no reactions in them, are, for them, as if non-existent.* Of the manifold vibrations which the ether is supposed to be incessantly undergoing, only certain vibrations affect the eye

At the time this was written, I had some fish ova in the course of development. Out of the same mass, and in the same vessel, all those which were supported by weed at a depth of half an inch from the surface, lived and developed; all those, without exception, that were at a depth of two to four inches, perished. In ordinary parlance, surely, nothing would be objected to in the phrase, "these ova were all in the same Medium"; the water was the same, the weed the same, the vessel the same; yet some difference of temperature and carbonic acid made all the difference between life and death. Another curious fact was observed; I removed eight of these ova with active embryos, and placed them in a large watch-glass containing a solution (one half per cent) of

as light; these constitute the Medium of Sight; the others are as if they were not. Only certain vibrations of the air affect the ear as Sound; to all other vibrations we are deaf; though ears of finer sensibility may detect them and be deaf to those which affect us.

55. "The external conditions of existence" is therefore the correct definition of the Medium. An animal may be surrounded with various foods and poisons, but if its organism is not directly affected by them they cannot be food or poison to it. An animal may be surrounded with carnivorous rivals, but if it is not adapted to serve them as food, or is too powerful to be attacked by them, they only indirectly enter into its Medium, by eating the food it would eat. The analogy is similar with anorganisms and their relation to their media. Every physical or chemical phenomenon depends on the concurrence of definite conditions namely, the substance which manifests the change, and the medium in which the change is manifested. Alter the medium, solid, liquid, or gaseous, change its thermal or electrical state, and the phenomenon is altered. But although similar alterations in the medium notoriously influence the organism, yet, because a great many variations in external conditions are unaccompanied by appreciable changes in the organism, there are biologists who regard this as a proof of Life being independent of physical and chemical laws; an error arising from their not recognizing the precise nature of organic conditions.

56. To give greater precision to the conception of a Medium it will be desirable to adopt the distinction much

bichromate of ammonia. In this acid the embryos lived and were active fifty-seven hours, although other embryos placed in a similar watch-glass containing pond-water, survived only forty hours. The non-effect of the acid was probably due to the non-absorption which nullifies the effect of certain virulent poisons when they are swallowed; but why the fish should live longer in the acid than in the simple water, I do not at all comprehend.