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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.)

(First Notice.)

Prof. Haeckel is well known as one of the most energetic workers and advanced thinkers among German biologists. For more than thirty years he has devoted himself to the study of the animal kingdom with especial reference to the theory of development, and he has perhaps done as much to extend and popularise that theory as Darwin himself. Besides a long series of publications in various departments of biology, he has written two great popular works—The History of Creation, in which the development of the whole animal and vegetable kingdom is systematically traced out, and the present volumes, which treat in more detail the entire history of man's evolution, both as an individual from the parental germ, and as an animal species from the most rudimentary form of individualised animal life through a progressive series of more and more specialised animal types.

The present work is intended to render the facts of human germ history and development accessible to the educated public. It is founded on the researches of the most eminent modern anatomists and embryologists—Baer, Kölliker, Schwann, Huxley, Weissmann, and Gegenbaur, together with Haeckel's own discoveries in the history and development of many of the lower animals. We can, therefore, hardly do otherwise than accept the facts as presented to us by our author, and though we may not always agree with the inferences he deduces from them, we can but feel that they are of the very highest importance, and that a careful study of them is absolutely essential before venturing to form definite conclusions as to man's nature, origin, or destiny. As the only way to give our readers any idea of this very remarkable work, we will endeavour to indicate the general nature of its contents, dwelling here and there on points of more especial interest and importance.

In the first chapter we are introduced to the "fundamental law of organic evolution," which is: that the history of the germ is an epitome of the history of the descent, or, more fully—

"That the series of forms through which the individual organism passes during its progress from the egg-cell to its fully developed state is a brief, compressed reproduction of the long series of forms through which the animal ancestors of that organism (or the ancestral form of its species) have passed from the earliest periods of so-called organic creation down to the present time."

The evolution of the individual is termed "Ontogeny," the evolution of the race (or, as he terms it, the tribe) "Phylogeny"—words which occur in almost every page of these volumes. It is then explained why the correspondence between these two kinds of development is not accurate, the reason being that the course of development of the embryo has been from time to time altered and much shortened, so that whole series of changes that have occurred in the successive modifications of animal forms have become compressed or altogether skipped in the evolution of the germ. The key to all these modifications and anomalies is to be found in heredity and adaptation; the former having kept up in the embryo the general type of earlier animal forms, the latter having so modified their details that the special ancestral type at each stage of development is often difficult to recognise, especially in the very early stages.

Chapter ii. gives an account of the early theories of development, such as the "preformation" and "encasement" of an endless series of organisms in each germ; and of the discoveries of Wolff, Harvey, Spallanzani, and others. Chapter iii. is devoted to the discoveries of Baer, which laid the foundation of the accurate knowledge of embryology. He first showed that the primitive germ-layers bend over till the edges meet, and thus form the primitive intestinal tube. He also first laid down the important law of evolution, which has been so extensively applied by Herbert Spencer—that it consists of a continually increasing differentiation of parts and tissues, combined with an increasing speciality of general form. In this chapter we first have the statement that the cells, of which all the tissues of the body are composed, "are independent living beings, the citizens of the state which constitute the entire multicellular organism." These cells increase by segmentation, dividing first into two, then into four, eight, sixteen, thirty-two, and so on, till an extensive stratum is formed called the germ-layer. This layer divides horizontally into two layers, and from these arise one or two intermediate layers. From the upper layer is formed the skin, and all its integuments, and also the brain, spinal marrow, and nervous system; from the lower layer is formed the intestinal canal and all its appendages—liver, lungs, &c.; while from the intermediate layers arise the muscles, blood, bones, and ligaments. This remarkable discovery was made by Remak, and has been confirmed by subsequent observers. The formation and separation of the primary germ-layers occurs throughout the whole animal kingdom above the Protozoa, and constitutes the most important fundamental fact of animal development.

Chapters iv. and v. give the history of Phylogeny or the theory of descent, from Lamarck to Darwin. Prof. Haeckel here maintains: that the struggle for existence in nature evolves new species without design, just as the will of man produces new varieties in cultivation with design, and, "that the

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evolution of the species or tribes contains, in the functions of heredity and adaptation, the determining cause of the evolution of individual organisms; or, briefly, Phylogeny is the mechanical cause of Ontogeny." We have here as it were the key-note of the work, the fundamental idea which the author never loses sight of. The science of rudimentary organs, which Haeckel terms "Dysteleology, or the Doctrine of Purposelessness," is here discussed, and a number of interesting examples are given, the conclusion being that they prove the mechanical or monistic conception of the origin of organisms to be correct, and the idea of any "all-wise creative plan" an ancient fable.

But all this is merely preliminary, and it is only in chapter vi. that we enter upon the real matter of the work, in a most interesting account of the egg-cell and the Amoeba. The popular idea of a cell (derived from those so easily seen in plants), as a closed sac or bladder with a defined solid envelope, is incorrect. The envelope is no essential part of the cell, but is in all cases a secondary formation. The modern definition of the cell is, that it is a small body, neither solid nor fluid, of an albuminous nature, and having enclosed in it a smaller roundish body, also albuminous. This is the nucleus, and it is this which is the essential characteristic of a living animal cell as distinguished from a mere lump of protoplasm. "Nucleus and protoplasm, the inner cell-kernel and the outer cell-slime, are the only two essential constituents of every genuine cell." Cells of various kinds are described and beautifully illustrated, and the nerve-cell is said to "possess the capacity to feel, to will, to think. It is a true mind-cell, an elementary organ of mental activity." These nerve-cells are highly complex in structure, whereas the egg-cell is in no way specialised; yet, from its active properties, we are obliged to infer a highly complex chemical composition of its protoplasmic substance, and a minute molecular structure, which are completely hidden from our eyes. Every cell is an independent organism. We see that it performs all the essential life-functions which the entire organism accomplishes. Every one of these little beings grows and feeds itself independently. It assimilates juices from without, absorbing them from the surrounding fluid; the naked cells can even take up solid particles at any point of their surface, and therefore eat without possessing either mouth or stomach. Each cell is also able to reproduce itself, and to increase. It is also able to move and creep about, if it has room for free motion, and is not prevented by a solid covering, while from its outer surface it sends out and draws back again finger-like processes, thus modifying its form. Cells from the watery humour of a frog's eye have been seen to move freely, and creep about just like the independent organisms termed Amoebae and Rhizopods. The young cell also has feeling, and is more or less sensitive, performing certain movements on the application of chemical and mechanical irritants. Thus we can trace in every single cell all the essential functions, the sum of which constitute the idea of life—feeling, motion, nutrition, reproduction.

Although there are even more simple organisms than cells, mere masses of living protoplasm without a nucleus, yet the cell as above described must be considered as the organic unit, the basis of our physiological idea of the elementary organism. For every animal without exception, from a sponge or worm up to man, originates in a primitive egg which is "an entirely simple, somewhat round, moving, naked cell, possessing no membrane, and consisting only of the nucleus and protoplasm." These egg-cells differ somewhat in size and form in different animals, but are essentially alike. Many organisms remain in this simple one-celled form, of which the Amoeba is the most familiar example. This creature, which most of our readers must have seen in a drop of water under the microscope, is important as being an example of the naked living cell, moving and feeding, and exhibiting all the signs of animal life, although a mere nucleated mass of protoplasm. It increases by division, the nucleus dividing first, and then the surrounding protoplasm distributes itself around the two new nuclei and parts into two distinct animals. Now it is a wonderful fact that the unfertilised eggs of some of the lower animals, as sponges and medusae, are absolutely undistinguishable from an Amoeba. Yet more, the blood-cells of many animals, and even the white corpuscles present in human blood, are exactly of the same character, moving, eating, and acting, just like Amoebae. For these reasons, the Amoeba is regarded as that one-celled organism which approaches nearest to the ancestral form of all animal life: and from a very similar cell every individual animal still originates.

The next chapter, on the processes of evolution and impregnation, is no less interesting and suggestive. The first step upward from the simple cell, would be the formation of groups of cells which remained attached to each other instead of parting as in the Amoeba. In this little community a division of labour would soon arise; some of the cells becoming specialised to absorb food, others to reproduce themselves, others to form protecting organs for the community, thus forming a distinct many-celled organism.

We have, then, a long discussion of the nature of reproduction, which is shown to be really a continuation of the growth of the individual; but we cannot see that any attempt is made to show how or why the sexes came to be differentiated as soon as the organisation became complex. This part of the subject is rather slurred over, and the whole process of fertilisation is said to be "extremely simple, and entirely without any special mystery. Essentially it consists merely in the fact that the male sperm-cell coalesces with the female egg-cell." The very mobile thread-shaped sperm-cells (spermatozoa) "find their way to the female egg-cells, penetrate the membrane of the latter by a perforating motion, and coalesce with the cell material." We hardly think that Prof. Haeckel's readers among the educated public will find this such a very simple matter. Considering that in the case of many marine animals these sperm-cells are discharged into the water, and have actually to seek the egg-cells and then penetrate their outer covering, it will be impossible to avoid the assumption that these apparently simple "cells" are not only living but intelligent organisms, endowed with a wonderful impulse to seek out and penetrate into eggs, thus destroying themselves in order to give birth to a new and higher being. However, when the two cells have coalesced, an important change takes place in the egg. Its nucleus disappears, and a new nucleus takes its place, which possesses the wonderful power of growing into the form of the parent organisms, however complex they may be. The egg-cell is now, therefore, a new formation, possessing in itself the vital activities of both parents combined.