Other facts and conclusions of importance have been contributed by various persons with whom ichthyology has been an incident rather than a matter of central importance.
The Fossil Fishes.[148]—The study of fossil fishes was begun systematically during the first decades of the nineteenth century, for it was then realized that of fossils of back-boned animals, fishes were the only ones which could be determined from early Palæozoic to recent horizons, and that from the diversity of their forms they could serve as reliable indications of the age of rocks. At a later time, when the evolution of vertebrates began to be studied, fishes were examined with especial care with a view of determining the ancestral line of the Amphibians. The earliest work upon fossil fishes is, as one would naturally expect, of a purely systematic value. Anatomical observations were scanty and crude, but as the material for study increased, a more satisfactory knowledge was gained of the structures of the various major groups of fishes; and finally by a comparison of anatomical results important light came to be thrown upon more fundamental problems.
The study of fossil fishes can be divided for convenience into three periods: (I) That which terminated in the magnum opus of Louis Agassiz; (II) that of the systematists whose major works appeared between 1845 and the recent publication of the Catalogue of Fossil Fishes of the British Museum (from this period date many important anatomical observations); and (III) that of morphological work, roughly from 1870 to the present. During this period detailed consideration has been given to the phylogeny of special structures, to the probable lines of descent of the groups of fossil fishes, and to the relationships of terrestrial to aquatic vertebrates.
First Period.—The Work of Louis Agassiz.—The real beginning of our knowledge of fossil fishes dates from the publication of the classic volumes of Agassiz, "Recherches sur les Poissons Fossiles (Neuchâtel, 1833-44)." There had previously existed but a fragmentary and widely scattered literature; the time was ripe for a great work which should bring together a knowledge of this important vertebrate fauna and the museums throughout Europe had been steadily growing in their collections of fossils. Especially ripe, too, since the work of Cuvier (1769-1832) had been completed and the classic anatomical papers of J. Müller (1802-56) were appearing. And Agassiz (1807-73) was eminently the man for this mission. At the age of one and twenty he had already mapped out the work, and from this time he devoted sixteen active years to its accomplishment. One gets but a just idea of the personality of Agassiz when he recalls that the young investigator while in an almost penniless position contrived to travel over a large part of Europe, mingle with the best people of his day, devote almost his entire time to research, employ draughtsmen and lithographers, support his own printing-house, and in the end publish his "Poissons Fossiles" in a fashion which would have done credit to the wealthiest amateur. With tireless energy he collected voluminous notes and drawings numberless; he corresponded with collectors all over Europe and prevailed upon them to loan him tons of specimens; in the meanwhile he collated industriously the early but fragmental literature in such works as those of de Blainville, Münster, Murchison, Buckland, Egerton, Redfield, W. C. Williamson, and others. Hitherto less than 300 species of fossil fishes were known; at the end of Agassiz's work about 900 were described and many of them figured.
It is easy to see that such a work made a ready basis of future studies. Doubtless, too, much is owing to the personal energy of Agassiz that such keen interest was focused in the collection and study of fossil fishes during the middle of the nineteenth century. The actual value of Agassiz's work can hardly be overestimated; his figures and descriptions are usually clear and accurate. And it is remarkable, perhaps, that in view of the very wide field which he covered that his errors are not more glaring and numerous. Upon the purely scientific side, however, one must confess that the "Poissons Fossiles" is of minor importance for the reason that as time has gone by it has been found to yield no generalizations of fundamental value. The classification of fishes advocated by Agassiz, based upon the nature of the scales, has been shown to be convenient rather than morphological. This indeed Agassiz himself appears to realize in a letter written to Humboldt, but on the other hand he regards his creation of the now discarded order of Ganoids, which was based upon integumental characters, as his most important contribution to the general study of ichthyology. And although there passed through his hands a series of forms more complete than has perhaps been seen by any later ichthyologist,[149] a series which demonstrates the steps in the evolution of the various families and even orders of fishes, he is nowhere led to such important philosophical conclusions as was, for example, his contemporary, Johannes Müller. And even to his last day, in spite of the light which palæontology must have given him, he denied strenuously the truth of the doctrine of evolution, a result the more remarkable since he has even given in graphic form the geological occurrence of the various groups of fishes in a way which suggests closely a modern phylogenetic table, and since at various times he has emphasized the dictum that the history of the individual is but the epitomized history of the race. The latter statement, which has been commonly attributed to Agassiz, is clearly of much earlier origin; it was definitely formulated by von Baer and Meckel, the former of whom even as early as 1834 pronounced himself a distinct evolutionist.
Ramsay Heatley Traquair.