THE LAST LINK

BIOGRAPHICAL SKETCHES

Jean Baptiste de Monet, Chevalier de Lamarck, was born on August 1, 1744, in Picardy, where his father owned land. Originally educated for the Church, he soon enlisted, and distinguished himself in active service. Owing to an accident affecting his health, the young Lieutenant gave up the military career, and, without means, studied medicine and natural sciences at Paris. In 1778 appeared his 'Flore française.' In 1793 he was appointed to a Chair of Zoology at the newly-formed Musée d'Histoire Naturelle. He had the misfortune to become gradually blind, and the last years of his life were spent amid straitened circumstances. He died in 1829.

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In 1794 Lamarck divided the whole animal kingdom into vertebrate and invertebrate animals, and founded successively the groups of Crustacea, Arachnida, Annelida, and Radiata. Between 1816 and 1822 he published his celebrated 'Histoire naturelle des Animaux sans Vertèbres.'

His most famous work is the 'Philosophie zoologique,' 1809.

Assuming the spontaneous origin of life, he propounded the doctrine that all animals and plants have arisen from low forms through incessant modifications and changes. In this respect he was in absolute opposition to Cuvier, who upheld the immutability of species, and did his best by absolute silence to suppress the spread of the new doctrine.

Lamarck has explained his views of transformism chiefly in the seventh chapter of the first volume of his 'Philosophie zoologique.'

Organisms strive to accommodate or adapt themselves to new circumstances, or to satisfy new requirements—e.g., climate, mode of[Pg 82] procuring food, escape from enemies. The continued function of parts of an organism changes the old and produces new organs. The acquirements are inherited by the offspring, and thus are produced the more complicated from simpler organisms. Continued disuse brings about degeneration and ultimate loss of an organ.

Lamarck consequently sees in the adaptability, or power of adaptation, which he assumes for all living matter the ultimate cause of variation; and, as he was certainly the first to point out that acquired characters are inherited by the progeny, he has given a working explanation of Evolution.

But his doctrine did not spread—partly because he was misunderstood. His theory, that a new want, by making itself felt, exacts from the animal new exertions, perhaps from parts hitherto not used, until the want is satisfied—this way of putting it sounds too teleological to explain the yearned-for change in a mechanical or natural way. Moreover,[Pg 83] many of his examples lacked the exact basis of experiment and observation necessary for their acceptance. Witness that of the neck of the giraffe,—a never-failing source of ridicule to men who cannot see the deeper purpose underlying the well-meant attempt at an explanation, which failed from want of complete knowledge of the intricate circumstances.

However, the theory of transformism was, so to speak, in the air; and various authors have written on the subject, filling the gap between Lamarck and Darwin, especially Goethe, Treviranus, Leopold von Buch, and Herbert Spencer. But it is Darwin's immortal merit to have opened our eyes by his theory of natural selection, which is, at least, the first attempt to explain some of the causes and incidents of organic Evolution in a natural mechanical way. Moreover, he was the first clearly to express the fundamental principles of the theory of descent, to elaborate what had been at best a general sketch of an ill-defined problem, and to enter[Pg 84] into detail, supported by a host of painstaking observations, the making of which had taken him half a lifetime. Darwin, without going further than cursorily into the causes of variation, argued as follows: We know that variations do occur in every kind of living creatures. Some of these variations lead to something, while others do not. An enormously greater number of animals and plants are born than reach maturity and can in their turn continue the race. What is the regulating factor? His answer is, The struggle for existence—in other words, the weeding out of the less fit, or rather of the owners of those variations which are not so well adapted to their surroundings.

For 'adapted' we had better read 'adaptable,' because a variation which does not answer, which cannot be made use of, or, still more notably, is a hindrance or disadvantage, does not become an adapted feature. There is often a confusion between adaptation as an accomplished fact, a feature,[Pg 85] or resultant condition, and adaptation as the mode of fitting the organism to, or making the best of, the prevailing surroundings or circumstances.

Étienne Geoffroy Saint-Hilaire was born in 1772 at Étampes, Seine-et-Oise. He was originally brought up for the Church; but when already ordained he attended lectures on natural science and medicine in Paris. He managed to get the place of assistant in the Musée d'Histoire Naturelle; he became Professor of Zoology in 1793, and took the opportunity of encouraging young Cuvier. Later he became Professor of Zoology of the Faculté des Sciences, and in 1818 he published his remarkable 'Philosophie anatomique.' He died in 1844.

He had conceived the 'unity of organic composition,' meaning that there is only one plan of construction,—the same principle, but varied in its accessory parts. In 1830, when Geoffroy proceeded to apply to the Inverte[Pg 86]brata his views as to the uniformity of animal composition, he found a vigorous opponent in Cuvier. Geoffroy, like Goethe, held that there is in Nature a law of compensation, or balancing of growth, so that if one organ take on an excess of development, it is at the expense of another part; and he maintained that, since Nature takes no sudden leaps, even organs which are superfluous in any given species, if they have played an important part in other species of the same family, are retained as rudiments, which testify to the permanence of the general plan of creation. It was his conviction that, owing to the conditions of life, the same forms had not been perpetuated since the origin of all things, although it was not his belief that existing species were becoming modified. Cuvier, on the other hand, maintained the absolute invariability of species, which, he declared, had been created with regard to the circumstances in which they were placed, each organ con[Pg 87]trived with a view to the function it had to fulfil,—thus putting the effect for the cause ('Encyclopædia Britannica,' 9th edition, vol. xxi., p. 171).

George Cuvier in the department of Doubs, which at that time belonged to Württemberg. He was educated at Stuttgart, and studied political economy. While acting as private tutor to a French family in France he followed his favourite pursuit, the study of natural sciences. Geoffroy Saint-Hilaire heard of him, and appointed him assistant in the department of comparative anatomy in the Musée d'Histoire Naturelle. In 1799 he was elected Professor of Natural History at the Collège de France, and soon after he became Perpetual Secretary of the Institut National. In 1831, a year before his death, Louis Philippe raised him to the rank of a peer of France.

Cuvier was the first to indicate the true[Pg 88] principle upon which the natural classification of animals should be based—namely, their structure. It is the study of the anatomy of the creatures and their comparison which affords the only sound basis of a classification. The work which had the greatest influence upon the scientific public is his 'Règne animal distribué d'après son Organisation,' 1817. The system which he propounded in this book gradually came to have almost world-wide fame, and, in spite of its many obvious deficiencies, still lingers in some of our most recent text-books.

A standard work is his 'Leçons d'Anatomie comparée,' and, in truth, he is the founder of that kind of comparative anatomy which was brought to such a high state by his pupil, the late Sir Richard Owen. Cuvier discovered the law of 'correlation of growth,' and was the first to apply this law to the reconstruction of animals from fragments: see his monumental work entitled 'Recherches sur les Ossemens fossiles,' 1812.

[Pg 89]

Cuvier, however, as a strict matter-of-fact man, was incapable of appreciating the speculative conclusions which were drawn by his contemporaries Saint-Hilaire and Lamarck. On the contrary, he firmly stuck to the doctrine of the immutability of species; and, in order to account for the existence of animals whose kind exists no longer, he invented the famous doctrine of successive cataclysms.

Karl Ernst von Baer was born in 1792 in Esthonia, studied at Dorpat and then at Würzburg, where Döllinger introduced him to comparative anatomy. For a few years he was a Privat-docent at Berlin; then he went to Königsberg as Professor of Zoology and Embryology. In 1834 he became an Academician at St. Petersburg, where for many years he was occupied with the most varied studies, chiefly geographical and ethnological. The last years of his long, active life he spent in contemplative retire[Pg 90]ment on his paternal estate, and he died at Dorpat in 1876.

While still at Würzburg he induced his friend Pander, a young man of means, to study the development of the chick; and Pander was the first to start the theory of the germinal layers from which all the organs arise. Baer, however, continued these researches in Königsberg, and after nine years' labour produced his epoch-making work, 'Ueber Entwicklungsgeschichte der Thiere: Beobachtung und Reflexion,' Königsberg, 1828. Nine years later he completed the second volume. He established upon a firm basis the theory of the germinal layers, and by further 'reflexions' arrived at the elucidation of some of the most fundamental laws of biology. For example, in the first volume he made the following prophetic statement: 'Perhaps all animals are alike, and nothing but hollow globes at their earliest developmental beginning. The farther back we trace their development, the more resemblance we find[Pg 91] in the most different creatures. And this leads to the question whether at the beginning of their development all animals are essentially alike, and referable to one common ancestral form. Considering that the "germ" (which at a certain stage appears in the shape of a hollow globe or bag) is the undeveloped animal itself, we are not without reason for assuming that the common fundamental form is that of a simple vesicle, from which every animal is evolved, not only theoretically, but historically.'

This statement is all the more wonderful when we consider that the cells, the all-composing individual units, were not discovered until ten years later.

In 1829 Baer discovered the human egg, and later the chorda dorsalis. In an address delivered in 1834, entitled 'The Most Universal Law of Nature in all Development,' he explained that only from a most superficial point of view can the various species be looked upon as permanent and immutable types;[Pg 92] that, on the contrary, they can be nothing but passing stages, or series of stages, of development, which have been evolved by transformation out of common ancestral forms.

Johannes Mueller, born at Coblenz in 1801, established himself as Privat-docent at Bonn, where in 1830 he became Professor of Physiology. In 1833 he accepted the Chair of Anatomy and Physiology at Berlin, where he died in 1858.

He was one of the most distinguished physiologists and comparative anatomists. By summarizing the labours and discoveries already made in the field of physiology, by reducing them to order, and abstracting the general principles, he became the founder of modern physiology. But he was scarcely less distinguished by his researches in comparative anatomy. His 'Vergleichende Anatomie der Myxinoiden,' in Abhandlungen der Berliner Akademie, 1835-45, and 'Ueber die Grenzen der Ganoiden' (ibid., 1846), are standard works of lasting value.

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Mueller exercised a stimulative influence as a teacher. Many well-known men—such as Helmholtz, Gegenbaur, Bruecke the physiologist, Guenther the zoologist, Virchow the pathologist, Koelliker and Haeckel—have been his pupils.

Rudolph Virchow Schievelbein, a small town in Eastern Pomerania. He studied medicine in Berlin as a pupil of Johannes Mueller, and went in 1849 to Würzburg, where, under the influence of Koelliker, and Leydig the pathologist, he laid the foundation of an entirely new branch of medical science—that of 'cellular pathology.' Since 1856 he has filled the principal Chair of Pathology at Berlin. In 1892 he received the Copley medal of the Royal Society.

'His contributions to the study of morbid anatomy have thrown light upon the diseases of every part of the body; but the broad and philosophical view he has taken of the[Pg 94] processes of pathology has done more than his most brilliant observations to make the science of disease.

'In pathology, strictly so called, his two great achievements—the detection of the cellular activity which lies at the bottom of all morbid as well as normal physiological processes, and the classification of the important group of new growths on a natural histological basis—have each of them not only made an epoch in medicine, but have also been the occasion of fresh extension of science by other labourers' (Proc. Royal Soc., 1892).

Virchow has not confined himself to medicine. He takes the keenest interest in anthropology and ethnology, on which subjects he has contributed many papers. Together with his colleagues Helmholtz the physicist, and Du Bois Reymond the physiologist, he has taken a leading place in the spreading of natural science; but, unfortunately, he did not take to the doctrine of[Pg 95] Evolution, and for the last thirty years has been its declared antagonist, rarely missing an opportunity of denouncing everything but descriptive anatomy and zoology as the unsound speculations of dreamers. This has on more than one occasion brought him into sharp conflict with Haeckel. His activity is astonishing, especially if it be remembered that Virchow has for many years been one of the most conspicuous leaders of the Progressists and Radicals in the German Parliament and Berlin town-council.

Edward Drinker Cope was born at Philadelphia, Pa. After studying at several Continental Universities, especially at Heidelberg, he became first Professor of Natural Science at Haverford College, and later Professor of Geology and Mineralogy. He died at an early age in 1897. As a member of various geological expeditions and other surveys, he explored chiefly Kansas, Wyoming, and Colorado; and he published many[Pg 96] most suggestive papers on the fossil vertebrate fauna of North America, and on classification especially of Amphibia and Reptiles.

Among works of a more general philosophical scope may be mentioned 'The Origin of the Fittest,' 1887, and his latest work, 'The Primary Factors of Organic Evolution,' 1896.

Albert von Koelliker became Professor of Anatomy at Würzburg. His earlier studies and discoveries contributed considerably to the systematic development of the cell theory. In 1844 he observed the division and further multiplication of the original egg cell. Next year he showed the continuity between nerve cells and nerve fibres in the Vertebrata; later, that the non-striped or smooth muscular tissue is composed of cellular elements. He demonstrated that the Gregarinæ are unicellular creatures. In 1852 he went with his younger friend Gegenbaur to Messina, where he studied especially[Pg 97] the development of the Cephalopoda (cuttlefishes and allies); and he produced a magnificent work on Alcyonaria, Medusæ, and other allied forms. He elucidated the development of the vertebral column, especially with reference to the notochord.

In 1848 he founded, together with Th. von Siebold, the famous Zeitschrift für wissenschaftliche Zoologie.

A standard work on mammalian embryology is his 'Entwicklungsgeschichte des Menschen und der höheren Thiere,' a text-book of which the second edition appeared in 1879.

At the anniversary meeting of 1897 he received the Copley medal, the highest honour which the Royal Society can bestow.

Carl Gegenbaur was born on August 21, 1826, in Bavaria. He studied medicine and kindred subjects in Würzburg, and as a pupil of Johannes Mueller in Berlin.

In 1852 he went with Koelliker to Messina[Pg 98] to study the structure and development of the marine fauna. Important papers on Siphonophora, Echinoderms, Pteropoda, and, later, Hydrozoa and Mollusca, were the result. Soon after his return he was offered the chair of Anatomy at Jena, and at this retired spot he produced his most important works, devoting himself more and more to the study of the Vertebrata. Since 1875 he has held the Chair of Anatomy at Heidelberg.

In 1859 he published his 'Principles of Comparative Anatomy'; but in 1870 he remodelled it completely, the theory of descent being the guiding principle. These 'Grundzüge' were followed by a somewhat more condensed 'Grundriss,' the second edition of which was published in 1878, and has been translated into French and English. In the meantime he had broken new ground by the development and treatment of certain problems concerning the composition and origin of the limbs, the shoulder-girdle and the skull, researches which are embodied[Pg 99] in his 'Untersuchungen zur vergleichenden Anatomie der Wirbelthiere,' 1864-65-72.

In 1883 he brought out a text-book on human anatomy. This also marked a new epoch, because for the first time, not only the nomenclature, but also the general treatment of human anatomy, was put upon a firm comparative anatomical basis. The success of this work is indicated by the fact that it reached the sixth edition in 1897.

Lastly, in 1898, appeared the first volume of what may be called his crowning work, 'Vergleichende Anatomie der Wirbelthiere.'

Gegenbaur is universally recognised, not only as the greatest living comparative anatomist, but also as the founder of the modern side of this science, by having based it on the theory of descent.

In 1896 he received from the Royal Society the Copley medal 'for his pre-eminence in the science of comparative anatomy or animal morphology.'

His marvellously powerful influence as a[Pg 100] teacher and investigator has made Heidelberg a centre whence many pupils have spread his teaching, and above all his method of research.

Ernst Heinrich Haeckel was born on February 16, 1834, at Potsdam. He carried out his academical studies alternately at Berlin and Würzburg, attracted by such men as Johannes Mueller, Koelliker, and Virchow. For years he was undecided what his career should be, whether that of botanist, collector, or geographical traveller. Certainly that of medicine attracted him least, although in deference to his father's wishes he qualified and settled down for a year's practice in Berlin. As he himself has told us, he might perhaps have proved rather successful as a physician, to judge from the fact that he did not lose a single patient. But 'I had only three patients all told, and the reason of this is perhaps that I had given on my plate the hours of consultation as from 5 to 6 a.m.'

[Pg 101]

During the year 1859 he travelled as medical man and artist in Sicily. In 1861 he was induced by Gegenbaur, whose acquaintance he had made in Würzburg, to establish himself as a Privat-docent for comparative anatomy in Jena. And there he has remained ever since, filling the Chair of Zoology, and having declined several much more tempting offers from the Universities of Würzburg, Vienna, Strassburg, and Bonn.

Within one year, 1865, he wrote the two volumes of his 'Generelle Morphologie der Organismen,' as he himself relates, in order to master his sorrow over the loss of his first wife. But he broke down, and went to the Canaries to recruit health and strength. The 'Morphologie,' which has long been out of print,[26] made scarcely any impression. It[Pg 102] was ignored, probably because he had placed the old-fashioned study of zoology and morphology upon a thoroughly Darwinistic basis.

On the advice of his friend Gegenbaur, he gave a more popularly written abstract of his 'Generelle Morphologie'—in fact, the substance of a series of his lectures—in the shape of his 'Natürliche Schöpfungsgeschichte.' This 'History of Natural Creation,' which in 1898 has reached the ninth edition (first edition translated into English in 1873), had the desired effect. So also had his 'Anthropogenie oder Entwicklungsgeschichte des Menschen,' the fourth edition of which appeared in 1891.

It was a lucky coincidence that Haeckel had just finished his preliminary academical studies, was entirely at leisure, and undetermined to which branch of natural science he should devote his genius, when Darwin's great work was given to the world. Haeckel embraced the new doctrine fervently, and, as Huxley was doing in England, he spread[Pg 103] it and fought for it with ever-increasing vigour in Germany.

With marvellous vigour and quickness of perception he applied the principles of Evolution or the theory of descent to the whole organic world, and not only opened entirely new vistas for the study of morphology, but also worked them out and fixed them. He was the first to draw up pedigrees of the various larger groups of animals and plants, filling the gaps by fossils or with hypothetical forms (the necessary existence of which he arrived at by logical deductions); and thus he reconstructed the first universal pedigree, a gigantic ancestral tree, from the simple unicellular Amœba to Man. Of course these pedigrees were entirely provisional, as he himself has over and over again avowed; but they are, nevertheless, the ideal which all systematists and morphologists working upon the basis of Evolution have since been seeking to establish.

Naturally he was vigorously attacked, not[Pg 104] only by anti-Darwinians, or rather anti-Evolutionists, but also by many of those who, having accepted the principle of transformism, ought to have known better. Perhaps they thought they did know better. Imperfections or mistakes in details of the grand attempt,—and these, naturally, were many,—were singled out as samples of the whole, which was ridiculed as the romance of a dreamer.

In the end, however, this hostility, narrow-minded and unfair in many respects, has done good to the cause. There has arisen an ever-increasing school of workers in favour of the new doctrine. Owing to renewed research, criticism, corrections in all directions, we now know considerably more about natural classification (and this is pedigree) than when Haeckel first opened out the whole problem.

Owing to his fearless mode of exposition, regardless of the indignant wrath which the new doctrine aroused in certain ecclesiastical[Pg 105] quarters, Haeckel bore the brunt of almost endless attacks, and had to write polemical essays. The result has been that friend and foe alike are now working on the lines which he has laid down; most of the ideas which he was the first to conceive, and to formulate by inventing a scientific terminology for them, have become important branches, or even disciplines, of the science.

Most morphologists of the younger generations now take these terms for granted, without remembering the name of their founder. It is, therefore, perhaps not quite superfluous to mention some of them:

Phylum, or stem, the sum total of all those organisms which have probably descended from one common lower form. He distinguished eight such phyla—Protozoa, Cœlenterata, Helminthes or Vermes, Tunicata, Mollusca, Articulata, and Vertebrata. The phyla are more or less analogous to 'super-classes,' large branches or 'circles,' or principal groups of other zoologists.

[Pg 106]

Phylogeny, the history of the development of the various phyla, classes, orders, families,and species.

Ontogeny, the history or study of the development of the individual, generally called embryology. In reality the scope of embryology is the ontogenetic study of the various species, and this branch of developmental study alone can be checked by direct, 'exact' observation, for the simple reason that the individuals alone are entities, while the species, genera, families, etc., are abstract ideas.

The ontogenesis of any given living organism is a short, condensed recapitulation of its ancestral history or of its phylogenesis. This is Haeckel's 'fundamental biogenetic law.'

A complete proof of the phylogeny of any creature would be given by the preservation of an unbroken series of all its fossil ancestors. Such a series will in most cases, for obvious reasons, always remain a desideratum. In a few cases, however, the desideratum is nearly[Pg 107] met: for example, the ancestral line of the one-toed digitigrade horse from a four-or five-toed plantigrade and still very generalized Ungulate is approaching completion.

Phylogenetic study has to rely upon other help. This is afforded by comparative anatomy and by the study of ontogeny. If the latter were a faithful, unbroken recapitulation of all the stages through which the ancestors have passed, the whole matter would be very simple; but we know for certain that in the individual development many stages are left out (or, rather, are hurried through, and are so condensed by short-cuts being taken that we cannot observe them), while other features which have been introduced obscure, and occasionally modify beyond recognition, the original course.

Again, the sequence of the appearance of the various organs is frequently upset (heterochronism). Some organs are accelerated in their development, while others,[Pg 108] which we know to be phylogenetically older, are retarded in making their reappearance in the embryo.

These disturbing or distorting newly introduced features or factors show themselves chiefly in connection with the embryonic conditions of growth—for example, yolk-sac, placenta, amnion. They all come within the category of cænogenesis: they are cænogenetic, while the true, undisturbed recapitulation is palingenetic.

Lastly, some features, so-called rudimentary or vestigial organs, instead of disappearing, are most tenacious in their recurrence, while others of originally fundamental importance scarcely leave recognisable traces, and are, so to speak, only hinted at during the embryonic growth of the creature we happen to study. Hence arises the philosophical study of 'Dysteleology.'

Among other terms invented by Haeckel, and now in general use, are Metamere, Metamerism, Cœlom, Gonochorism, Gastrula,[Pg 109] Metazoa, Gnathostomata, Acrania, Craniota, and Amniota.

Hitherto we have dealt with his general work only, a résumé of which he gave for many years in a course of thirty lectures before an audience composed of 'all sorts and conditions of men.' Students of biology and of medicine side by side with theologians, incipient and ordained, jurists, political economists, and philosophers, crowded his lecture-room during the 'seventies to hear the master explaining the 'natural history of creation' or the mysteries of anthropogenesis. Another course of eighty lectures during the winter semester was, and still is, devoted to a systematic treatment of zoology, while practical classes are reserved for the more select.

His winning personality and fascinating eloquence, combined with a clear and concise delivery, have gained the enthusiastic admiration of many a student who went to the quiet University town in order to learn with his own ears and eyes.

[Pg 110]

List of Separate Publications by Professor Haeckel.

'Biologische Studien. I.: Studien ueber die Moneren und andere Protisten.' Leipzig, 1870 (out of print). He was the first to make observations on the natural history of the Monera, living bits of protoplasm, devoid even of a nucleus—e.g., Protogenes primordialis, Protomyxa aurantiaca.

'Monographie der Radiolarien.' Berlin, 1862-88. With 171 plates.

'Entwicklungsgeschichte der Siphonophoren.' Utrecht, 1869.

'Plankton-Studien. Vergleichende Untersuchungen ueber die Bedeutung und Zusammensetzung der pelagischen Fauna und Flora.' Jena, 1880.

'Metagenesis und Hypogenesis von Aurelia aurita.' Jena, 1881.

'Monographie der Geryoniden oder Ruesselquallen.' Leipzig, 1865.

'Generelle Morphologie der Organismen.' 2 vols. Berlin, 1866.

[Pg 111]

'Anthropogenie oder Entwicklungsgeschichte des Menschen,' 1874; 4th edition, 1891.

'Natuerliche Schoepfungs-Geschichte.' 2 vols. Berlin, 1st edition, 1868; 9th edition, 1898. This work has been translated into most European languages (the first edition in English, under the title 'Natural History of Creation' in 1873; the eighth in 1892).

'Monographie der Kalkschwaemme.' 3 vols. Berlin, 1872 (out of print). With the subtitle, 'An Attempt to solve analytically the Problem of the Origin of Species.' In this work, illustrated by sixty plates, he showed that the Calcispongia are individually so yielding, so adaptive to external influences, that it is practically impossible to break up the whole group into anything like satisfactory species or genera. According to predilection, we can distinguish either 1 genus with only 3 species, or 3, 21, 43 genera, with 21, 111, 181, or 289 species respectively.

In this work, in 1872, Haeckel established[Pg 112] the homology of the two primary layers, ecto- and endoderm, throughout the Metazoa. The attempt to do the same for the four secondary layers, as made in the second part of his 'Gastræa-theory,' failed. It caused an enormous amount of research, hitherto without a satisfactory solution of the problem.

'Studien zur Gastræa-Theorie.' Jena, 1874. The transformation of the single primitive egg-cell by cleavage into a globular mass of cells (Morula)—which latter, becoming hollow (and then known as the Blastula), turns ultimately by invagination or by delamination into the Gastrula—is a series of processes which applies to all Metazoa. The Gastrula is, therefore, the ancestral form of the Metazoa; and the Gastræa-theory, founded by Haeckel, throws light, on the one hand, upon the mystery of the phyletic connection of the various animal groups, while, on the other hand, it connects the Metazoa, or multicellular organisms, with the lowest Protozoa. We come to this conclusion because[Pg 113] the Gastrula arises from and passes through stages which exist as independent, permanent organisms among the Protozoa.

Needless to say this Gastræa-theory has been violently attacked in detail, with the result that various modifications of the Gastrula, until then undreamed of, have become known.

'Monographie der Medusen.' Jena, 1879-81. With 72 coloured plates.

'Reports on the Scientific Results of the Voyage of H.M.S. Challenger.' With 230 plates:

1. Deep-sea Medusæ. 1881.
2. Radiolaria. 1887.
3. Siphonophoræ. 1888.
4. Deep-sea Keratosa. 1889.

A short holiday spent on the coasts of the Red Sea produced the volume 'Arabische Korallen' (Berlin, 1876); and a longer trip to Ceylon has been described in 'Indische Reisebriefe,' of which the third edition[Pg 114] appeared in 1893. The English translation (1883) is entitled 'A Visit to Ceylon.'

'Monism as connecting Religion and Science: the Confession of Faith of a Man of Science.' 1894.

Haeckels latest work is the 'Systematische Phylogenie' (Berlin, 1896), three volumes dealing with Protistæ and Plants, Invertebrata and Vertebrata. They contain the author's views on the natural system of the organic world, both living and extinct. Notable in the work are the many reconstructions of ancestral forms which, provided Evolution is true, must have existed—hypothetical until they, or something like them, are found in a fossil state. Everybody who works systematically, and upon the basis of Evolution, does, sometimes unconsciously, reconstruct such links, although he may perhaps not see the necessity, or have the courage to fix his vision, by assigning to it all those attributes or characters which are indicated by deductions from comparative anatomy, palæontology, and embryology.

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