Chapter 12: 1843-1846: Aet. 36-39.
- Completion of fossil fishes. -- followed by fossil fishes of the old Red Sandstone. -- review of the later work. -- identification of fishes by the skull. -- renewed correspondence with Prince Canino about journey to the United States. -- change of plan owing to the interest of the King of Prussia in the expedition. -- correspondence between Professor Sedgwick and Agassiz on development theory. -- final scientific work in Neuchatel and Paris. -- publication of ‘Systeme Glaciaire.’ -- short stay in England. -- sails for United States.
In 1843 the ‘Recherches sur les Poissons Fossiles’ was completed, and fast upon its footsteps, in 1844, followed the author's ‘Monograph on the Fossil Fishes of the Old Red Sandstone, or the Devonian System of Great Britain and Russia,’ a large quarto volume of text, accompanied by forty-one plates. Nothing in his paleontological studies ever interested Agassiz more than this curious fauna of the Old Red, so strange in its combinations that even well-informed naturalists had attributed its fossil remains to various classes of the animal kingdom in turn, and, indeed, long  remained in doubt as to their true nature. Agassiz says himself in his Preface: ‘I can never forget the impression produced upon me by the sight of these creatures, furnished with appendages resembling wings, yet belonging, as I had satisfied myself, to the class of fishes. Here was a type entirely new to us, about to renter (for the first time since it had ceased to exist) the series of beings; nor could anything, thus far revealed from extinct creations, have led us to anticipate its existence. So true is it that observation alone is a safe guide to the laws of development of organized beings, and that we must be on our guard against all those systems of transformation of species so lightly invented by the imagination.’ The author goes on to state that the discovery of these fossils was mainly due to Hugh Miller, and that his own work had been confined to the identification of their character and the determination of their relations to the already known fossil fishes. This work, upon a type so extraordinary, implied, however, innumerable and reiterated comparisons, and a minute study of the least fragments of the remains which could be procured. The materials were chiefly obtained in Scotland; but Sir Roderick Murchison also contributed his own  collection from the Old Red of Russia, and various other specimens from the same locality. Not only on account of their peculiar structure were the fishes of the Old Red interesting to Agassiz, but also because, with this fauna, the vertebrate type took its place for the first time in what were then supposed to be the most ancient fossiliferous beds. When Agassiz first began his researches on fossil fishes, no vertebrate form had been discovered below the coal. The occurrence of fishes in the Devonian and Silurian beds threw the vertebrate type back, as he believed, into line with all the invertebrate classes, and seemed to him to show that the four great types of the animal kingdom, Radiates, Mollusks, Articulates, and Vertebrates, had appeared together.1 ‘It is henceforth demonstrated,’ says Agassiz, ‘that the fishes were included in the plan of the first organic combinations which made the point of departure for all the living inhabitants of our globe in the series of time.’ In his opinion this simultaneity of appearance, as well as the richness and variety displayed by invertebrate classes from the beginning,  made it2 ‘impossible to refer the first inhabitants of the earth to a few stocks, subsequently differentiated under the influence of external conditions of existence.’ . . . He adds:3 ‘I have elsewhere presented my views upon the development through which the successive creations have passed during the history of our planet. But what I wish to prove here, by a careful discussion of the facts reported in the following pages, is the truth of the law now so clearly demonstrated in the series of vertebrates, that the successive creations have undergone phases of development analogous to those of the embryo in its growth and similar to the gradations shown by the present creation in the ascending series, which it presents as a whole. One may consider it as henceforth proved that the embryo of the fish during its development, the class of fishes as it at present exists in its numerous families, and the type of fish in its planetary history, exhibit analogous phases through which one may follow the same creative thought like a guiding thread in the study of the connection  between organized beings.’ Following this comparison closely, he shows how the early embryonic condition of the present fishes is recalled by the general disposition of the fins in the fishes of the Old Red Sandstone, and especially by the caudal fin, making the unevenly lobed tail, so characteristic of these ancient forms. This so called heterocercal tail is only known to exist, as a permanent adult feature, in the sturgeons of to-day. The form of the head and the position of the mouth and eyes in the fishes of the Old Red were also shown to be analogous with embryonic phases of our present fishes. From these analogies, and also from the ascendency of fishes as the only known vertebrate, and therefore as the highest type in those ancient deposits, Agassiz considered this fauna as representing ‘the embryonic age of the reign of fishes;’ and he sums up his results in conclusion in the following words: ‘The facts, taken as a whole, seem to me to show, not only that the fishes of the Old Red constitute an independent fauna, distinct from those of other deposits, but that they also represent in their organization the most remarkable analogy with the first phases of embryonic development in the bony fishes of our epoch, and a no less  marked parallelism with the lower degrees of certain types of the class as it now exists on the surface of the earth.’ It has been said by one of the biographers of Agassiz,4 in reference to this work upon the fishes of the Old Red Sandstone: ‘It is difficult to understand why the results of these admirable researches, and of later ones made by him, did not in themselves lead him to support the theory of transformation, of which they seem the natural consequence.’ It is true that except for the frequent allusion to a creative thought or plan, this introduction to the Fishes of the Old Red might seem to be written by an advocate of the development theory rather than by its most determined opponent, so much does it deal with laws of the organic world, now used in support of evolution. These comprehensive laws, announced by Agassiz in his ‘Poissons Fossiles,’ and afterward constantly reiterated by him, have indeed been adopted by the writers on evolution, though with a wholly different interpretation. No one saw more clearly than Agassiz the relation which he first pointed out, between the succession of animals of the same type in time and the phases of their embryonic  growth to-day, and he often said, in his lectures, ‘the history of the individual is the history of the type.’ But the coincidence between the geological succession, the embryonic development, the zoological gradation, and the geographical distribution of animals in the past and the present, rested, according to his belief, upon an intellectual coherence and not upon a material connection. So, also, the variability, as well as the constancy, of organized beings, at once so plastic and so inflexible, seemed to him controlled by something more than the mechanism of selfadjust-ing forces. In this conviction he remained unshaken all his life, although the development theory came up for discussion under so many various aspects during that time. His views are now in the descending scale; but to give them less than their real prominence here would be to deprive his scientific career of its true basis. Belief in a Creator was the keynote of his study of nature. In summing up the comprehensive results of Agassiz's paleontological researches, and especially of his ‘Fossil Fishes,’ Arnold Guyot says:5— ‘Whatever be the opinions which many  may entertain as to the interpretation of some of these generalizations, the vast importance of these results of Agassiz's studies may be appreciated by the incontestable fact, that nearly all the questions which modern paleontology has treated are here raised and in great measure solved. They already form a code of general laws which has become a foundation for the geological history of the life-system, and which the subsequent investigations of science have only modified and extended, not destroyed. Nowhere did the mind of Agassiz show more power of generalization, more vigor, or more originality. The discovery of these great truths is truly his work; he derived them immediately from nature by his own observations. Hence it is that all his later zoological investigations tend to a common aim, namely, to give by farther studies, equally conscientious but more extensive, a broader and more solid basis to those laws which he had read in nature and which he had proclaimed at that early date in his immortal work, “ Poissons Fossiles.” Let us not be astonished that he should have remained faithful to these views to the end of his life. It is because he had seen that he believed, and such a faith is not easily shaken by new hypotheses.’ 
The pressure of work upon his various publications detained Agassiz at home during the summer of 1844. For the first time he was  unable to make one of the glacial party this year, but the work was carried on uninterruptedly, and the results reported to him. Meantime his contemplated journey to the United States flitted constantly before him.
The next letter announces a new aspect of the projected journey. In explanation, it should be said that finding Agassiz might be prevented by his poverty from going, the prince had invited him to be his guest for a summer in the United States.
The pleasant plan so long meditated was not to be fulfilled. The prince was obliged to defer the journey and never accomplished it. This was a great disappointment to Agassiz. ‘Am I then to go without you,’ he writes; ‘is this irrevocable? If I were to defer my departure till September would it then be possible for you to leave Rome? It would be  too delightful if we could make this journey together. I wish also, before starting, to review everything that has been done of late in paleontology, zoology, and comparative anatomy, that I may, in behalf of all these sciences, take advantage of the circumstances in which I shall be placed. . . . Whatever befalls me, I feel that I shall never cease to consecrate my whole energy to the study of nature; its all powerful charm has taken such possession of me that I shall always sacrifice everything to it; even the things which men usually value most.’ Agassiz had determined, before starting on his journey, to complete all his unfinished works, and to put in order his correspondence and collections, including the vast amount of specimens sent him for identification or for his own researches. The task of ‘setting his house in order’ for a change which, perhaps, he dimly felt to be more momentous than it seemed, proved long and laborious. From all accounts, he performed prodigies of work, but the winter and spring passed, and the summer of 1845 found him still at his post. Humboldt writes him not without anxiety lest his determination to complete all the tasks he had undertaken, including the Nomenclator,  should involve him in endless delays and perplexities.
 The following correspondence with Professor Adam Sedgwick is of interest, as showing his attitude and that of Agassiz toward questions which have since acquired a still greater scientific importance.
In the ‘Revue Suisse’ of April, 1845, there is a notice of the course of lectures to which reference is made in the above letter.
A numerous audience assembled on the 26th of March for the opening of a course by Professor Agassiz on the “Plan of creation.” It is with an ever new pleasure that our public come together to listen to this savant, still so young and already so celebrated. Not content with pursuing in seclusion his laborious scientific investigations, he makes a habit of communicating, almost annually, to an audience less restricted than that of the Academy the general result of some of his researches. All the qualities to which Mr. Agassiz has accustomed his listeners were found in the opening prelude; the fullness and freedom of expression which give to his lectures the character of a scientific causerie; the dignified  ease of bearing, joined with the simplicity and candor of a savant who teaches neither by aphorisms nor oracles, but who frankly admits the public to the results of his researches; the power of generalization always based upon a patient study of facts, which he knows how to present with remarkable clearness in a language that all can understand. We will not follow the professor in tracing the outlines of his course. Suffice it to say that he intends to show in the general development of the animal kingdom the existence of a definite preconceived plan, successively carried out; in other words, the manifestation of a higher thought,—the thought of God. This creative thought may be studied under three points of view: as shown in the relations which, spite of their manifold diversity, connect all the species now living on the surface of the globe; in their geographical distribution; and in the succession of beings from primitive epochs until the present condition of things.The summer of 1845 was the last which Agassiz passed at home. It was broken by a short and hurried visit to the glacier of the Aar, respecting which no details have been preserved. He did not then know that  he was taking a final leave of his cabin among the rocks and ice. Affairs connected with the welfare of the institution in Neuchatel, with which he had been so long connected, still detained him for a part of the winter, and he did not leave for Paris until the first week in March, 1846. His wife and daughters had already preceded him to Germany, where he was to join them again on his way to Paris, and where they were to pass the period of his absence, under the care of his brother-in-law, Mr. Alexander Braun, then living at Carlsruhe. His son was to remain at school at Neuchatel. It was two o'clock at night when he left his home of so many years. There had been a general sadness at the thought of his departure, and every testimony of affection and respect accompanied him. The students came in procession with torchlights to give him a parting serenade, and many of his friends and colleagues were also present to bid him farewell. M. Louis Favre says in his Memoir, ‘Great was the emotion at Neuchatel when the report was spread abroad that Agassiz was about to leave for a long journey. It is true he promised to come back, but the New World might shower upon him such marvels  that his return could hardly be counted upon. The young people, the students, regretted their beloved professor not only for his scientific attainments, but for his kindly disposition, the charm of his eloquence, the inspiration of his teaching; they regretted also the gay, animated, untiring companion of their excursions, who made them acquainted with nature, and knew so well how to encourage and interest them in their studies.’ Pausing at Carlsruhe on his journey, he proceeded thence to Paris, where he was welcomed with the greatest cordiality by scientific men. In recognition of his work on the ‘Fossil Fishes’ the Monthyon Prize of Physiology was awarded him by the Academy. He felt this distinction the more because the bearing of such investigations upon experimental physiology had never before been pointed out, and it showed that he had succeeded in giving a new direction and a more comprehensive character to paleontological research. He passed some months in Paris, busily occupied with the publication of the ‘Systeme Glaciaire,’ his second work on the glacial phenomena. The ‘Etudes sur les Glaciers’ had simply contained a resume of all the researches undertaken upon the Alpine  fields of ice and the results obtained up to 1840, inclusive of the author's own work and his wider interpretation of the facts. The ‘Systeme Glaciaire’ was, on the contrary, an account of a connected plan of investigation during a succession of years, upon a single glacier, with its geodetic and topographic features, its hydrography, its internal structure, its atmospheric conditions, its rate of annual and diurnal progress, and its relations to surrounding glaciers. All the local phenomena, so far as they could be observed, were subjected to a strict scrutiny, and the results corrected by careful comparison, during five seasons. As we have seen, and as Agassiz himself says in his Preface, this band of workers had ‘lived in the intimacy of the glacier, striving to draw from it the secret of its formation and its annual advance.’ The work was accompanied by three maps and nine plates. In such a volume of detail there is no room for picturesque description, and little is told of the wonderful scenes they witnessed by day and night, nothing of personal peril and adventure. This task concluded, he went to England, where he was to spend the few remaining days previous to his departure. Among the  last words of farewell which reached him just as he was leaving the Old World, little thinking then that he was to make a permanent home in America, were these lines from Humboldt, written at Sans Souci:
So closed this period of Agassiz's life. The next was to open in new scenes, under wholly different conditions. He sailed for America in September, 1846.