Page:EB1911 - Volume 20.djvu/637

 The greatest generalization of this second period, however, was that partly prepared for by d’Orbigny, as will be more fully explained later in this article, and clearly expressed by Agassiz—namely, the law of repetition of ancestral stages of life in the course of the successive stages of individual development. This law of recapitulation, subsequently termed the “biogenetic law” by Ernest Haeckel, was the greatest philosophic contribution of this period, and proved to be not only one of the bulwarks of the evolution theory but one of the most important principles in the method of palaeontology.

On the whole, as in the case of vertebrate palaeontology, the pre-Darwinian period of invertebrate palaeontology was one of rather dry systematic description, in which, however, the applications of the science gradually extended to many regions of the world and to all divisions of the kingdom of invertebrates.

Beginning with the publication of Darwin’s great works, “Narrative of the Surveying Voyages of H. M.S. ‘Adventure’ and ‘Beagle’&#8198;” (1839), and “On the Origin of Species by Means of Natural Selection” (1859).—A review of the two first classic works of Charles Robert Darwin (1809–1882) and of their influence proves that he was the founder of modern palaeontology. Principles of descent and other applications of uniformitarianism which had been struggling for expression in the writings of Lamarck, St Hilaire and de Blainville here found their true interpretation, because the geological succession, the rise, the migrations, the extinctions, were all connected with the grand central idea of evolution from primordial forms.

A close study of the exact modes of evolution and of the philosophy of evolution is the distinguishing feature of this period. It appears from comparison of the work in the two great divisions of vertebrate and invertebrate palaeontology made for the first time in this article that in accuracy of observation and in close philosophical analysis of facts the students of invertebrate palaeontology led the way. This was due to the much greater completeness and abundance of material afforded among invertebrate fossils, and it was manifested in the demonstration of two great principles or laws: first, the law of recapitulation, which is found in its most ideal expression in the shells of invertebrates; second, in the law of direct genetic succession through very gradual modification. It is singular that the second law is still ignored by many zoologists. Both laws were of paramount importance, as direct evidence of Darwin’s theory of descent, which, it will be remembered, was at the time regarded merely as an hypothesis. Nevertheless, the tracing of phylogeny, or direct lines of descent, suddenly began to attract far more interest than the naming and description of species.

''The Law of Recapitulation. Acceleration. Retardation.''—This law, that in the stages of growth of individual development (ontogeny), an animal repeats the stages of its ancestral evolution (phylogeny) was, as we have stated, anticipated by d’Orbigny. He recognized the fact that the shells of molluscs, which grow by successive additions, preserve unchanged the whole series of stages of their individual development, so that each shell of a Cretaceous ammonite, for example, represents five stages of progressive modification as follows: the first is the période embryonnaire, during which the shell is smooth; the second and third represent periods of elaboration and ornamentation; the fourth is a period of initial degeneration; the fifth and last a period of degeneration when ornamentation becomes obsolete and the exterior smooth again, as in the young. D’Orbigny, being a special creationist, failed to recognize the bearing of these individual stages on evolution. Alpheus Hyatt (1838–1902) was the first to discover (1866) that these changes in the form of the ammonite shell agreed closely with those which had been passed through in the ancestral history of the ammonites. In an epoch-making essay, On the Parallelism between the Different stages of Life in the individual and those in the entire group of the Molluscous Order Tetrabranchiata (1866), and in a number of subsequent memoirs, among which Genesis of the Arietidae (1889)

and Phylogeny of an Acquired Characteristic (1894) should be mentioned, he laid the foundations, by methods of the most exact analysis, for all future recapitulation work of invertebrate palaeontologists. He showed that from each individual shell of an ammonite the entire ancestral series may be reconstructed, and that, while the earlier shell-whorls retain the characters of the adults of preceding members of the series, a shell in its own adult stage adds a new character, which in turn becomes the pre-adult character of the types which will succeed it; finally, that this comparison between the revolutions of the life of an individual and the life of the entire order of ammonites is wonderfully harmonious and precise. Moreover, the last stages of individual life are prophetic not only of future rising and progressing derivatives, but in the case of senile individuals of future declining and degradational series.

Thus the recapitulation law, which had been built up independently from the observations and speculations on vertebrates by Lorenz Ofen (1779–1851), Johann Friedrich Meckel (1781–1833), St Hilaire, Karl Ernst von Baer (1792–1876) and others, and had been applied (1842–1843) by Karl Vogt (1817–1895) and Agassiz, in their respective fields of observation, to comparison of individual stages with the adults of the same group in preceding geological periods, furnished the key to the determination of the ancestry of the invertebrates generally.

Hyatt went further and demonstrated that ancestral characters are passed through by successive descendants at a more and more accelerated rate in each generation, thus giving time for the appearance of new characters in the adult. His “law of acceleration” together with the complementary “law of retardation,” or the slowing up in the development of certain characters (first propounded by E. D. Cope), was also a philosophic contribution of the first importance (see fig. 6 and Plate III., fig. 7).

In the same year, 1866, Franz Martin Hilgendorf (1839–) studied the shells of Planorbis from the Miocene lake basin underlying the present village of Steinheim in Württemberg, and introduced the method of examination of large numbers of individual specimens, a method which has become of prime importance in the science. He discovered the actual transmutations in direct genetic series of species on the successive deposition levels of the old lake basin. This study of direct genetic series marked another great advance, and became possible in invertebrate palaeontology long before it was introduced among the vertebrates. Hyatt, in a re-examination of the Steinheim deposits, proved that successive modifications occur at the same level as well as in vertical succession. Melchior Neumayr (1845–1890) and C. M. Paul similarly demonstrated genetic series of Paludina (Vivipara) in the Pliocene lakes of Slavonia (1S75).

''The Mutations of Waagen. Orthogenesis.''—In 1869 Wilhelm Heinrich Waagen (1841–1900) entered the field with the study of Ammonites subradiatus. He proposed the term “mutations” for the minute progressive changes of single characters in definite directions as observed in successive stratigraphic levels. Even when seen in minute features only he recognized them as constant progressive characters or “chronologic varieties” in