The most novel results comprised in this work were: first, the remodeling of the classification of the whole type of fishes, fossil and living, and especially the separation of the Ganoids from all other fishes, under the rank of a distinct order; second, the recognition of those combinations of reptilian and bird-like characters in the earlier geological fishes, which led the author to call them prophetic types; and third, his discovery of an analogy between the embryological phases of the higher present fishes and the gradual introduction of the whole type on earth, the series in growth and the series in time revealing a certain mutual correspondence. As these comprehensive laws have thrown light upon other types of the animal kingdom beside that of fishes, their discovery may be said to have advanced general zoology as well as ichthyology.
The Introduction presents, as it were, the prelude to this vast chapter of natural history in the simultaneous appearance of the four great types of the animal kingdom: Radiates, Mollusks, Articulates, and Vertebrates. Then comes the orderly development of the class by which the vertebrate plan was first expressed, namely, the fishes. Underlying all its divisions and subdivisions, is the average expression of the type in the past and present; the Placoids and Ganoids, with their combination of reptilian and fishlike features, characterizing the earlier geological epochs, while in the later the simple bony fishes, the Cycloids and Ctenoids, take the ascendancy. Here, for the first time, Agassiz presents his "synthetic or prophetic types," namely, early types embracing, as it were, in one large outline, features afterward individualized in special groups, and never again reunited. No less striking than these general views of structural relations are the clearness and simplicity with which the distribution of the whole class of fishes in relation to the geological formations, or, in other words, to the physical history of the earth, is shown. In reading this introductory chapter, one familiar with Agassiz as a public teacher will almost hear his voice marshaling the long procession of living beings, as he was wont to do, in their gradual introduction upon the earth. Indeed, his whole future work in ichthyology, and one might almost say in general zoology, was here sketched.
The technicalities of this work, at once so comprehensive in its combinations and so minute in its details, could interest only the professional reader, but its generalizations may well have a certain attraction for every thoughtful mind. It treats of the relations, anatomical, zoological, and geological, between the whole class of fishes, fossil and living, illustrated by numerous plates, while additional light is thrown on the whole by the revelations of embryology.
"Notwithstanding these striking differences," says the author in the opening of the fifth chapter on the relations of fishes in general, "it is none the less evident to the attentive observer that one single idea has presided over the development of the whole class, and that all the deviations lead back to a primary plan, so that even if the thread seem broken in the present creation, one can reunite it on reaching the domain of fossil ichthyology."* (* Volume 1 chapter 5 pages 92, 93.)
Having shown how the present creation has given him the key to past creations, how the complete skeleton of the living fishes has explained the scattered fragments of the ancient ones, especially those of which the soft cartilaginous structure was liable to decay, he presents two modes of studying the type as a whole; either in its comparative anatomy, including in the comparison the whole history of the type, fossil and living, or in its comparative embryology. "The results," he adds, "of these two methods of study complete and control each other." In all his subsequent researches indeed, the history of the individual in its successive phases went hand in hand with the history of the type. He constantly tested his zoological results by his embryological investigations.
After a careful description of the dorsal chord in its embryological development, he shows that a certain parallelism exists between the comparative degrees of development of the vertebral column in the different groups of fishes, and the phases of its embryonic development in the higher fishes. Farther on he shows a like coincidence between the development of the system of fins in the different groups of fishes, and the gradual growth and differentiation of the fins in the embryo of the higher living fishes.* (* "Recherches sur les Poissons Fossiles", volume 1 chapter 5 page 102.) "There is, then," he concludes, "as we have said above, a certain analogy, or rather a certain parallelism, to be established between the embryological development of the Cycloids and Ctenoids, and the genetic or paleontological development of the whole class. Considered from this point of view, no one will dispute that the form of the caudal fin is of high importance for zoological and paleontological considerations, since it shows that the same thought, the same plan, which presides to-day over the formation of the embryo, is also manifested in the successive development of the numerous creation which have formerly peopled the earth." Agassiz says himself in his Preface: "I have succeeded in expressing the laws of succession and of the organic development of fishes during all geological epochs; and science may henceforth, in seeing the changes of this class from formation to formation, follow the progress of organization in one great division of the animal kingdom, through a complete series of the ages of the earth." This is not inconsistent with his position as the leading opponent of the development or Darwinian theories. To him, development meant development of plan as expressed in structure, not the change of one structure into another. To his apprehension the change was based upon intellectual, not upon material causes. He sums up his own conviction with reference to this question as follows:* (* "Recherches sur les Poissons Fossiles" volume 1 chapter 6 pages 171, 172. "Essay on the Classification of Fishes.") "Such facts proclaim aloud principles not yet discussed in science, but which paleontological researches place before the eyes of the observer with an ever-increasing persistency. I speak of the relations of the creation with the creator. Phenomena closely allied in the order of their succession, and yet without sufficient cause in themselves for their appearance; an infinite diversity of species without any common material bond, so grouping themselves as to present the most admirable progressive development to which our own species is linked,—are these not incontestable proofs of the existence of a superior intelligence whose power alone could have established such an order of things?. . ."
"More than fifteen hundred species of fossil fishes, which I have learned to know, tell me that species do not pass insensibly one into another, but that they appear and disappear unexpectedly, without direct relations with their precursors; for I think no one will seriously pretend that the numerous types of Cycloids and Ctenoids, almost all of which are contemporaneous with one an other, have descended from the Placoids and Ganoids. As well might one affirm that the Mammalia, and man with them, have descended directly from fishes. All these species have a fixed epoch of appearance and disappearance; their existence is even limited to an appointed time. And yet they present, as a whole, numerous affinities more or less close, a definite coordination in a given system of organization which has intimate relations with the mode of existence of each type, and even of each species. An invisible thread unwinds itself throughout all time, across this immense diversity, and presents to us as a definite result, a continual progress in the development of which man is the term, of which the four classes of vertebrates are intermediate forms, and the totality of invertebrate animals the constant accessory accompaniment."
The difficulty of carrying out comparisons so rigorous and extensive as were needed in order to reconstruct the organic relations between the fossil fishes of all geological formations and those of the present world, is best told by the author.* (* "Recherches sur les Poissons Fossiles" volume 1. Addition a la Preface.) "Possessing no fossil fishes myself, and renouncing forever the acquisition of collections so precious, I have been forced to seek the materials for my work in all the collections of Europe containing such remains; I have, therefore, made frequent journeys in Germany, in France, and in England, in order to examine, describe, and illustrate the objects of my researches. But notwithstanding the cordiality with which even the most precious specimens have been placed at my disposition, a serious inconvenience has resulted from this mode of working, namely, that I have rarely been able to compare directly the various specimens of the same species from different collections, and that I have often been obliged to make my identification from memory, or from simple notes, or, in the more fortunate cases, from my drawings only. It is impossible to imagine the fatigue, the exhaustion of all the faculties, involved in such a method. The hurry of traveling, joined to the lack of the most ordinary facilities for observation, has not rendered my task more easy. I therefore claim indulgence for such of my identifications as a later examination, made at leisure, may modify, and for descriptions which sometimes bear the stamp of the precipitation with which they have been prepared."
It was, perhaps, this experience of Agassiz's earlier life which made him so anxious to establish a museum of comparative zoology in this country,—a museum so abundant and comprehensive in material, that the student should not only find all classes of the animal kingdom represented within its walls, but preserved also in such numbers as to allow the sacrifice of many specimens for purposes of comparison and study. He was resolved that no student should stand there baffled at the door of knowledge, as he had often done himself, when shown the one precious specimen, which could not be removed, or even examined on the spot, because unique.