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The Evolution of Man: A Popular Exposition of the Principal Points of Human Ontogeny and Phylogeny. From the German of Ernst Haeckel, Professor in the University of Jena. In Two Volumes. (C.Kegan Paul & Co.)

(Second Notice.)

In chapter viii. the succeeding stages of development of the fertilised germ are traced throughout the animal kingdom, with special reference to the gastrula, a primitive animal form which Prof. Haeckel believes can be discovered in the early stages of all animals, and must, therefore, be considered as representing one of their earliest ancestral types. In the corals the successive cleavage of the parent cell leads to the formation of a globular mass, called from its appearance the mulberry germ. It consists of a single layer of cells in close contact, forming a hollow ball filled with a clear liquid. There next occurs an extraordinary process of inversion. A groove forms at one point by the sinking in of the cellular layer; this groove deepens and widens till it forms a cup-shaped cavity, and at last the two sides come together, forming a double-walled cup. The month of this cup then narrows, vibrating threads are formed on the outer surface, and the gastrula germ is then complete. The cells of the outer and inner surfaces have now assumed a different form, size, and appearance; the inside is a stomach, the outside a skin. A great variety of animals go through this peculiar stage of development with but slight differences—such as zoophytes, worms, star-fish, crustacea, molluscs, and the lowest of vertebrates, the Amphioxus or lancelet. In all the higher vertebrates this process of gastrulation is highly modified; but in every group, even up to man, Prof. Haeckel maintains that it exists, and can be traced in its various forms, of which he gives very instructive illustrations. The essential feature of gastrulation is that the mass of cells formed by cleavage becomes differentiated into two groups or layers, from one of which is ultimately formed the outer skin, from the other the intestinal organs; hence these are termed the animal and the vegetative germ-layers respectively. Many of the lowest animals—such as some of the Polyps—remain throughout their life in the gastrula stage, their whole body being composed of only two cell-strata or layers; hence the important conclusion is arrived at that all the higher animals, including man, which in the first stages of their individual evolution pass through a two-layered structural stage or gastrula form, must have descended from a primaeval, simple parent form of like structure, to which Prof. Haeckel gives provisionally the name of Gastraea, or primitive intestinal animal.

In the next chapter the scheme of classification founded on this gastraea-theory is explained. From the gastraea developed in one direction the zoophytes—such as sponges, corals, medusae, &c.; in another direction the worms. The zoophytes are a side branch, while the worms form the main stem of the animal tree from which all the other great classes—molluscs, insects, and vertebrates—have been evolved. The vertebrate nature of man is next discussed, and the structure of the ideal primitive vertebrate explained in great detail; and then comes an account of the various parts and organs which arise from each of the four germ-layers into which the two primitive layers divide at a very early period. This is very remarkable and instructive. From the first or outer layer are formed, not only the skin and all its appendages, but also the central nerve system. This first develops from the outer surface of the epidermis, and only at a later stage moves inward so as to be surrounded and protected by bone and muscle. The organ of the mind, therefore, is a development of the outer skin where alone it could be in contact with external nature. The kidneys also arise from the skin, and subsequently take their place deep within the body. From the second layer arise the skeleton and all the chief muscles of the trunk and limbs. From the third arises the entire vascular system, the heart and blood vessels, the blood, and the muscular coating of the intestines; while from the fourth or inner layer arises the intestinal canal proper and its appendages, such as the lungs, liver, and salivary glands.

In chapter x. the process of development of the gastrula into a perfect vertebrate organism is described in detail, and illustrated by numerous diagrams. The extraordinary processes by which the external cell-layer bends inwards, forms loops and folds which then become detached to form internal organs, such as the spinal cord and kidneys, are made very intelligible by means of elaborate figures, in which the parts that arise from each germ-layer are distinguished by different colours. The two following chapters carry on this examination into further details, describing the development of the vertebral column, and the successive appearance of the more important organs in the human embryo.

In the next two chapters (xiii. and xiv.) we enter upon another branch of the subject—the origin of the vertebrate type. We have first a full description of the structure of the Amphioxus and the Ascidians. The former is universally admitted to be the lowest existing type of vertebrate animal, while the latter were formerly classed as Mollusca, but are now believed by many biologists to be extremely modified forms of the most rudimental vertebrate. In appearance they are shapeless lumps, hardly like animals, but looking more like fleshy potatoes. In the Italian fish-markets they are known as "sea-fruit." When caught they feebly contract their body and spirt out a little water: hence they have been called Sea-squirts. They vary in size from a quarter of an inch to a foot long, and they are found in the seas of all parts of the world. They are fixed by a kind of foot or root to the sea-bottom; at the top is a round opening which serves as a mouth, and on one side is a smaller opening. The mouth opens into a large latticed gill-sac into which water is drawn and discharged by the side opening, and through the gills the food also passes into the stomach, the intestine bending upward and opening into the cavity which surrounds the gill sac. The outer covering is tough and leather-like, while there is no trace of any internal skeleton.

Here there is absolutely nothing of the vertebrate structure, though there are some peculiarities in the formation of the gill-sac which resemble the same organ in the Amphioxus. But, strange to say, in the earlier stages of the development of the Ascidian there appear unmistakeable signs of resemblance to the vertebrate. A free-swimming long-tailed larva is developed from the gastrula, and in this there appears a medullary tube and also a notochord or rudimentary vertebral column, exactly as in the Amphioxus. Rudimentary sense organs also appear, according to Kowalewsky, who has studied the history of this animal; but then its progressive development ceases. It sinks to the bottom of the sea and becomes fixed, the tail with the notochord degenerates and is cast off, and the tailless body, by retrograde metamorphosis, loses all its vertebrate characteristics and becomes a shapeless sac, as already described. While in the Amphioxus the medullary tube develops into a complete spinal marrow, in the Ascidian it shrinks away to an insignificant nerve-ganglion situated just above the gill-body. These curious facts are held to prove that the Ascidians really represent a degenerated branch of the ancestral vertebrate, very near the point of its actual origin.

From this original form it is not difficult to understand the development of the Amphioxus, which is universally admitted to be a true vertebrate, though of very low type. It is usually classed as a low form of fish; but Prof. Haeckel holds this to be a great error. By the complete absence of a skull and of even the rudiments of limbs, and by its excessively simple internal structure, it is said to be further removed from fishes than fishes are from man. He therefore looks upon the Amphioxus with special veneration, as the only living animal which can enable us to form an approximate conception of our earliest vertebrate ancestors.

We now come to the second volume, which is devoted to a more special examination of the line of animal ancestry that has ultimately culminated in the development of man, and to a detailed account of the development of the various parts and organs of the human frame, with constant references to the comparative embryology of other animals. We have here much repetition of facts and arguments already given in the first volume, and shall therefore only briefly notice a few points which seem to call for remark.

Prof. Haeckel seems quite unable to appreciate the extreme imperfection of the geological record, and the absolute worthlessness of its negative evidence as regards the life of the earliest periods. He speaks of the inhabitants of our planet consisting exclusively of aquatic forms down to the Silurian period (p. 10), and that we may infer with tolerable certainty that no land animals then existed (p. 115), quite regardless of the fact that the enormous deposits of this period are all marine, and are therefore not likely to contain remains of land animals, and also of the equally important

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fact that the sandstones, grits, shales, and limestones of which they are composed necessitate extensive continents from the denudation of which they were formed, and that it is in the highest degree improbable that these continents were lifeless wastes. Equally improbable are his suppositions that mammalia originated in the Trias (p. 144), and placental mammals in the Tertiary epoch (p. 15). Considering that even in the Lower Eocene most of the orders and many of the family groups of placental mammals are well differentiated, most English biologists would look very far back into the Mesozoic epoch for the first differentiation of the placental and the implacental divisions.

The celebrated Bathybius—the living protoplasm of the ocean depths, which was first described by Prof. Huxley from specimens preserved in spirit and given up by him when the living animal was sought for in vain during the Challenger expedition—is resuscitated by Haeckel on the authority of Dr. Emil Bessil, who is said to have obtained it alive from a depth of 550 feet in Smith's Sound. It is often said that the protoplasm of Amoeba and other simple organisms is only apparently structureless owing to the insufficiency of our optical powers; but Prof. Haeckel remarks that the experiment of feeding these animals with solid coloured particles which can be seen passing through their substance irregularly in all directions shows that they are really structureless in the sense in which we always use the word as applied to molar, not molecular, structure. When we consider that these structureless particles of slime yet exhibit, as Prof. Haeckel himself tells us, all the phenomena of life, "even the mental phenomena," his theory, which he is never tired of putting forward, that all the phenomena of the organic no less than of the inorganic world are due to "mechanical laws" does not seem to throw much light on the matter. He is equally confident that our

"highly purposive and admirably constituted sense-organs have developed without premeditated aim; that they originated by the same mechanical process of Natural Selection, by the same constant interaction of Adaptation and Heredity by which all the other purposive contrivances of the animal organisation have been slowly and gradually evolved during the Struggle for Existence."

Yet Prof. Haeckel is not a materialist. He maintains that the materialistic philosophy, which asserts that the vital phenomena are due to the properties of matter, is as false as the opposite spiritualistic philosophy, which declares that active force precedes or causes matter. Both, he maintains, are dualistic, and therefore both are equally false. The monistic philosophy which he upholds as alone tenable can as little believe in force without matter as in matter without force. So far, we might not perhaps differ greatly from him; but when he goes on to say, "the 'spirit' and 'mind' of man are but forces which are inseparably connected with the material substance of our bodies," and to argue that thinking-force and motive-force are equally functions of the body, he seems to confuse radically distinct conceptions, by the use of the misleading word "forces" as applicable to thought or emotion. His final conclusion is—

"that in the entire history of the evolution of man no other active forces have been at work than in the rest of organic and inorganic nature. All the forces at work there can be reduced at last to growth—to that fundamental function of evolution by which the forms of inorganic as well as of organic bodies originate. Growth, again, itself rests on the attraction and repulsion of like and unlike particles. It has given rise to Man and to Ape, to Palm and to Alga, to crystal and water."

Although I have endeavoured to give an account of some of the more suggestive portions of this very remarkable work, a notice such as this can afford no conception of the wonderful variety and complexity, or of the intensely interesting nature, of the subjects it discusses. There is probably no book in any language which gives so full, so clear, and so perfectly intelligible an account of the earlier stages of the development of animals. The phenomena described are, as compared with the later stages of development, simple and easily followed, but it is impossible to exaggerate their importance; and as enabling any intelligent person to obtain a correct knowledge of the facts of this wonderful history in its earlier, and a correct conception of their general outlines and bearing in their later and more complex stages, the work is one of the most important in the English language. Its faults are diffuseness of style and complexity of general arrangement, and a competent editor would be able to condense it into one half the bulk without curtailing it of any important matter. It is nevertheless most acceptable even as it is, and should be studied by everyone who wishes to appreciate the full meaning of the familiar saying, that "we are fearfully and wonderfully made."