Popular Science Monthly/Volume 80/March 1912/The Four Periods in the Development of the Modern Zoological System
|THE FOUR PERIODS IN THE DEVELOPMENT OF THE MODERN ZOOLOGICAL SYSTEM|
IN 1758 when Linnæus published the epoch-making tenth edition of his "Systema Naturæ" the science of zoology was in a backward condition, having made but little progress for a long period of time. Some important advances, it is true, had been made by the generation immediately preceding that event. Trembley and Peysonnel had proved the animal nature of Hydra and of corals; Linck and Klein had increased the knowledge of the obscure group of echinoderms; Réaumur had continued the brilliant researches of Swammerdam on insects. The discovery of microscopic animals, also, in the preceding century, had opened up new vistas, into which, however, the scientists of the day saw as yet but dimly. Zoology was still, notwithstanding these things, a very crude descriptive science, in which but few fruitful attempts at comparative or philosophical studies had been made.
The cause of this failure to progress rapidly was not the lack of able and earnest zoologists in the preceding ages or even the absence of new discoveries, but the chaotic condition of the zoological classification and nomenclature, which stood in the way of the recognition of the true relationships of animals. A chaos could not become the basis of a system of philosophy. When thus in 1758 Linnæus introduced his fully developed binomial system and arranged all the animals then known to science according to its rules into classes, orders, genera and species he provided the key which should unlock the mysteries of zoology as a science, and disclosed the wonders it contained.
The essential feature of this system and that which was new at the time was the giving to each species of animals of two names, instead of one, or of several, one of which was the specific name and the other the name of the next higher subdivision in the classification, the genus. The other important features were the precisions of the terminology employed, which enables the author to characterize a species in a few words, and the natural arrangement of the classification in which the position of each species indicates the degree of its genetic relationship to all the others.
It is true that predecessors of Linnæus had anticipated many features of his system. The idea of a species was already well fixed before his time and efforts were made to characterize those then known and the new ones which were constantly being discovered. But the names given were often complex and cumbersome and no uniformity existed between the systems of terminology of different authors. Also the custom of giving two or more Latin names to a species was frequently in vogue, but a binomial system, with the definite relation of the specific to the generic name, was new. The genus, which gives the clue to the natural affinities of the animal, was peculiarly Linnæus's invention.
Attempts had also been made by Ray and Klein and other advanced thinkers to form a system which should express the natural relationships of animals, but such attempts were generally not understood or followed and most authors still employed unnatural methods of arranging them. Many still followed Pliny and grouped animals according to the environmental conditions surrounding them, placing those together having similar methods of life, as land animals, freshwater animals, marine animals, flying animals, etc. Within each group the species were often arranged in alphabetical order.
Linnæus's system was very quickly accepted by the scientific world and went into universal use, and modern zoology may in a very real sense be said to begin with the year 1758.
So radical, however, was Linnæus's reform that neither the superiority of his system nor the simplicity of his terminology would probably have been sufficient thus to procure its adoption if they had not been proposed by a man of his great fame and commanding position in the world. Linnæus was considered by his contemporaries, because of his numerous and important contributions to science and his eminence as a teacher in the University of Upsala, as the greatest naturalist of all time. His importance was indicated by the phrase in vogue: Deus creavit; Linnæus disposuit.
The immediate acceptance of the Linnæan classification had the same effect upon the study of animals and plants in his day as that of Darwin's theory of natural selection had almost exactly one hundred years later. It gave a tremendous impetus to every branch of biological investigation and started a new era. Systematic zoology, morphology, physiology and experimental zoology all attracted able investigators who studied them with feverish activity. Comparative studies first became possible, as now the facts of the science were for the first time arranged in something like an orderly and natural manner, and the next generation saw the rise of the sciences of comparative anatomy, paleontology and comparative embryology, and also the first modern speculations on the blood relationships and the evolution of living things.
All these things gave a new importance to zoology and raised it from the position it had occupied of a mere annex to medicine to the dignity of an independent science.
Linnæus divided the animal kingdom into six classes: Mammalia, Aves, Amphibia, Pisces, Insecta and Vermes. The knowledge of this last class, which included all invertebrate animals except the arthropods, was in a very confused state, and one of the chief objects of the many able zoologists of the generation immediately following him was to remedy this condition. The men whose services were greatest in this direction were O. F. Müller, Lamarck and Cuvier. In 1794 Lamarck first distinguished the vertebrates from the invertebrates and subdivided the latter group into the five classes of Mollusca, Insecta, Vermes, Echinodermata and Polypi. Thus a long step was taken towards modernizing the system and this early effort of Lamarck may be said to be the first modern classification of animals. He, in his later works, further subdivided the invertebrate types until he had ten, the fundamental idea at the basis of his classification being that the various groups of animals constitute a single ascending series which begins with the lowest and ends with the highest. This principle of the unity of the type found a wide acceptance among the naturalists of that time and was based upon the law: Natura non facit saltum.
In 1812 Cuvier published his division of the animal kingdom into four branches or types and in 1817 his great work "Le Règne Animal" which established the second great reform of the system and was destined to exert an influence only second to that of Linnæus's "Systema Naturæ" upon the study of animals and the development of the system. In these works Cuvier controverted the principle of the unity of type. among animals and taught that instead of one four distinct and permanent types prevail. It was upon these four types that he based his four fundamental branches of the animal kingdom: Vertebrata, Articulata, Mollusca and Zoophyta or Radiata.
A comparison of this classification with that of Linnæus will show what a tremendous advance had been made in the development of the system in the half century separating them. The group of animals which had benefited most in this general advance was probably the Mollusca, which was Cuvier's special field of research. The lowest group in Cuvier's system, as that in Linnæus's, was the one about which the least was known, the Zoophyta or Radiata being made up of several distinct and heterogeneous groups of animals which bore no near relationships to one another.
This condition led to an active investigation during the generation immediately following of all the lower animals and a very large number of works of fundamental importance appeared. Rudolphi studied the parasite worms, Tiedemann and L. Agassiz the anatomy and Johannes Müller the development of echinoderms, Ehrenberg the microscopic animals, Eschscholtz, Sars and others jellyfish and polyps. The knowledge of these two latter groups was also very much extended as the result of various scientific expeditions which were sent out by the French, English, Russian and American governments to different parts of the world, especially to the tropical oceans. Of these voyages perhaps the most interesting were that of the Russian ship Rurik from 1818 to 1820, in which Chamisso and Eschscholtz went as naturalists and discovered the alternation of generation of Salpa, that of the English ship Beagle between 1831 and 1835 with Darwin as naturalist, and the American expedition under Captain Wilkes between 1838 and 1842 with James Dwight Dana as the principal naturalist.
The influence of all these investigations, and also of the newly established cellular theory of the structure of plants and animals, on the development of the zoological system led to the third great reform of the latter. In 1845 von Siebold subdivided Cuvier's fourth type, the Zoophyta or Radiata, into three types or phyla, the Protozoa, Zoophyta and Vermes, confining thus the term Zoophyta to the truly radiate animals. He also broke up Cuvier's second type Articulata, removing the Annelida to the new phylum Vermes and creating another new phylum for the Crustacea, Arachnida, Myriapoda and Insecta which he called the Arthropoda. Two years later E. Leuckart broke up the Zoophyta, subdividing it into the phyla Echinodermata and Coelenterata, and emphasized the isolated position of the Protozoa, and a little later Milne-Edwards added still another new type or phylum, the Molluscoidea, in which he included the Bryozoa, Brachiopoda and Tunicata. The animal kingdom was thus in 1850 subdivided into eight phyla, the Protozoa, Echinodermata, Vermes, Arthropoda, Molluscoidea, Mollusca and Vertebrata, an arrangement which is still found in many text books.
Darwin's "Origin of Species" was published in 1859 and the fourth and last important reform of the zoological system of classification was the direct consequence of the doctrines therein promulgated. The theory of the common descent and blood relationship of all animals which Darwin taught was at variance with Cuvier's theory of fixed types and in harmony with Lamarck's theory of the essential unity of the animal kingdom, and was first employed by Haeckel as the basis of a system of classification. In 1877 he called attention to the need of placing the entire system on an evolutionary basis and at the same time subdivided the animal kingdom into the two great groups of the Protozoa and the Metazoa, and the latter into the two great groups of the Coelenterata and the Cœlomata. In still more recent times other authors, notably Hatschek, following Haeckel's lead, have carried the subdivision still further on the same basis. The old idea of types, however, has a very tenacious life and is still the basis of the classification of animals in most text-books—and probably rightly so. For most animals, notwithstanding their ultimate relationships with one another, can as a matter of fact be grouped in a number of distinct types or phyla, each of which has a characteristic plan of structure. Cuvier's belief, however, that these types are fixed and isolated creations, has long since been abandoned.
Very important has been the formation in recent times of the phylum Chordonia or Chordata, which brings under the same subdivision all the animals possessing the essential characteristics of the vertebrate type. The formation of this phylum has been due to the fundamental researches of Kowalevsky, who in 1866, 1867 and 1871 gave the first detailed and accurate description of the anatomy of Balanoglossus and also the first detailed accounts of the embryology of ascidians and of Amphioxus, showing that these animals are related to one another and to vertebrates. The term Chordonia was introduced in 1874 by Haeckel to include the Tunicata, Amphioxus and the Vertebrata and the terms Urochorda and Cephalochorda by Lankester in 1878 for the Tunicata and Amphioxus. In 1884 Bateson, on the basis of his researches on the American form Balanoglossus aurantiacus, added the Enteropneusta to the Chordata and proposed the term Hemichorda.
The system of zoological classification was thus fixed some twenty or thirty years ago and has undergone no important changes in its larger features since. This is not true, however, of many of the subordinate and smaller of its groups, the arrangement of which changes from time to time as the knowledge of the relationships of the animals composing them increases. We find this to be especially true of certain low animals which seem to be isolated side branches of the ancestral tree, the origin of which from the main stem is still obscure.
Each of these four distinct periods of reform of the modern zoological system has been inaugurated by one or two eminent men of great constructive powers who have been able to see deeper into the significance of facts than their predecessors and contemporaries and to interpret rightly those which they have gathered. The first reform was started by Lamarck and the second by Cuvier, the third by von Siebold and Leuckart and the fourth by Darwin and Haeckel.