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The Animal World in its Present State.
IN order to approach the doctrine of Descent, and to prepare for its necessity, we purpose next to pass in review a main part of its object,—the present condition of the animal world in its general outlines. Organisms, as every one may see, are distinguished from animate. bodies by a certain mutability of existence; a sequence and alternation of phenomena, combined with constant absorption and expulsion of matter. These changes, which are ultimately molecular motions, and are therefore calculable, definable, and susceptible of investigation, take place in particles in a state of saturation-that is to say, soaked in water and aqueous fluids; and this peculiar, yet purely mechanical condition, suffices for the explanation and comprehension of many of the necessary phenomena of life. Experience shows that this capacity for saturation, and this mobility, essentially characterize the combinations of carbon; and the sum of these motions and displacements, of which a great part has already been susceptible of mathematically certain investigation, is termed Life.
Now it is impossible to resist the impression that there are simple and composite, lower and higher, living beings; and we likewise feel, more strongly than words will express, a certain antithesis between the plant and
the animal. Poetically regarded, the plant is the passive organism as described by Rückert:
"I am the garden flower
And meekly bide the hour,
The antithesis of the passive, quiescent plant and the pugnacious active animal diminishes, however, as we descend in the scale of both kingdoms. The more highly developed animal evinces its animal nature by the vivacity with which it reacts to external influences and excitations. In the lower animals the phenomena of life assume a more vegetal character, and in many groups of lower beings, which Haeckel has recently comprised under the name Protista, we see the processes of metamorphosis of tissue, nutrition, and reproduction taking place, indeed, but in a manner so simple and undifferentiated, that we too must attribute to these beings a neutral position betwixt plants and animals. We gain the conviction that the roots of the vegetal. and animal kingdoms are not completely sundered, but, to continue the simile, mèrge imperceptibly into each other by means of a connective tissue. In this intermediate kingdom the much derided "primordial slime" (Urschleim) of the natural philosophers has regained its honourable position. Many thousand cubic miles of the sea-bottom consist of a slime or mud composed in part of manifestly earthy inorganic portions, in part of
*"Ich bin die Blum' im Garten
peculiarly formed chalk corpuscles, still perhaps ambiguous in their nature (the Coccoliths and Rhabdoliths), and finally, which is the main point, of an albuminous substance which is alive.
This living slime, the so-called Bathybius, does not even exhibit individuality, or the definiteness of a separate existence; it resembles the shapeless mineral substances, each particle of which bears the characteristics of the whole.
The conception of an organism as a being composed of various parts, with various offices or functions, and appearing under a definite form gradually developed, is in our day so inherent and intuitive, that it is only with great exertion that we are able to accommodate ourselves to the idea of a living mass either absolutely formless and undefined, or defined arbitrarily and accidentally. Let any one, who either cannot or will not do this, pause for a moment to contemplate another simple being-for instance, Haeckel's "Protamoeba." A small albuminous mass increases by the absorption of nutriment, and by the appropriation of matter, until it reaches a certain circumference, and then propagates itself by spontaneous fission into two equal parts. To our means of observation, these and similar beings are the simplest organisms devoid of organs. While accentuating the limits of research as restricted by inadequate means of observation, we maintain the validity. of Rollet's retort,' that our reason cannot properly admit such homogeneous organisms, performing all the functions of life solely by means of their atomic constitution; that we are dealing with the still utterly unknown structure of the molecules formed by the
aggregation of atoms; and that if Brücke says, "Apart from the molecular structure, we must also ascribe to living cells another structure of a different order of complexity, and this is what we denote by organization," we must likewise ascribe this yet unknown combination to the Monera of Haeckel.
But independently of this complexity of the molecular structure, it is of extreme importance to the investigation of animate nature to have become acquainted with bodies which present the simplest structure to the assisted eye, and to anatomical research. The substance which characterizes them is found again in plants as well as in animals; and plants and animals must now be regarded as two classes of organisms, in which the processes of self-preservation and reproduction have, in different ways, assumed the character of a higher complexity and development, by the differentiation of the originally homogeneous substance into various morphological structures and organs.
As we shall have another opportunity of expressing an opinion in regard to the beginnings of animal life, and its points of contact with protista and plants, we shall transfer ourselves from the dubious boundary line into the midst of the animal kingdom, in order to master our subject by sifting and arranging it.
The first impression of infinite variety is succeeded by another, that there are lower and higher animals. On this point complete harmony prevails. For if, from teleological considerations, invalid in our eyes, the nature of every creature were said to be perfect, that is, in correspondence with its purpose or idea, every one takes it for granted and self-evident that a standard of excellence
exists, without taking account of the scale by which it. rises or sinks. This standard will, however, soon be made manifest by the comparison of a lower with a higher animal. Let us select the fresh-water polype and the bee.
The little animal, several lines in length, which in our waters usually lives adhering to a plant, is a hollow cylinder, of which the body-wall is formed of two layers of cells, a layer of muscles, and a supporting membrane, which gives consistency to the whole, and may be compared to a skeleton. The mouth is surrounded by arms of similar construction, and varying in number from four to six. The surface of the body is studded with numerous little stinging vesicles, which by their contact stun any smaller animalculæ straying within the reach of the polype, and render them an easy prey. This is, in a few words, the construction of the animal. It possesses no arterial system, no special respiratory apparatus; the functions of the nerves and the sensory organs are performed by the individual parts of the surface. Reproduction is usually effected by the budding of gemmules, which fall off at maturity, but occasionally also by the produce of very simple sexual organs.
On the other hand, hours do not suffice to describe the structure of a bee. Even externally, its body, which possesses so highly complicated a structure, promises a rich development of the interior. The manducatory apparatus can be rendered comprehensible only by comparison with the oral organs of the whole insect world. The various divisions of the alimentary canal are each provided with special glands. The rich. psychical life, all the actions which imply intelligence,