All the mountains which we see now have not existed from the beginning of things. Mountains do not, however, grow as do plants. The stones of which mountains are composed have only a certain analogy with the bones of animals, but have no similarity in structure or in origin, nor have they the same function and purpose. Mountain ranges, or chains of mountains as some prefer to call them, do not always run in certain directions, though this has sometimes been claimed. Such claims correspond neither to reason nor to observation. Mountains may be very much disturbed in the course of years. Mountain peaks rise and fall somewhat. Chasms open and shut here and there in them, and though there are those who pretend that it is only the credulous who will accept the stories of such happenings, there is no doubt that they have been established on trustworthy evidence.
In the course of his observations in Italy, Stensen had seen many mussel shells, which had been gathered from various layers of the earth's surface. With regard to the shells themselves, he said that there could be no doubt that they had come as the excretion of the mantle of the mussel, and that the differences that could be noted in them were in accordance with the varying forms of these animals. He pointed out, however, that some of the mussel shells found in [{158}] strata of rock were really mussel shells in every respect as regards the material of which they were composed as well as their interior structure and their external form, so that there could be no possible question of their origin. On the other hand, a certain number of the so-called mussel shells were not composed of the ordinary materials of which such shells are usually made up; but had indeed only the external form of genuine shells. Stensen considered, however, that even these must be regarded as originating in real mussel shells, the original substance having been later on replaced by other material. He explained this replacement process in very much the same way that we now suggest the explanation of various processes of petrification. There is no doubt that in this he went far beyond his contemporaries, and pointed out very clearly what was to be the teaching of generations long after his own.
The same principles he applied to mussel shells, Stensen considered must have their application also to all other portions of animal bodies, teeth, bones, whole skeletons, and even more perishable animal materials that might be found buried in the earth's strata. His treatment of the question of the remains of plants was quite as satisfactory as that of the animals. He distinguished between the impressions of plants, the petrification of plants, the carbonization of plants, and then dwelt somewhat on the tendency of certain minerals to form dendrites, that is, branching [{159}] processes which look not unlike plants. He pointed out how easy it is to be deceived by these appearances, and stated very clearly the distinction between real plants and such simulated ones.
It will be scarcely necessary for us to apologize for having given so much space to Stensen's work on geology. Many distinguished scientists, however, have insisted that no greater advance at the birth of a science was ever made than that which Stensen accomplished in his geological work. Hoffman says that after carefully studying the work, he has come to the conclusion that of the successors of Stensen, no student of the mountains down to Werner's day had succeeded in comprehending so many fruitful points of view in geology. None of his great successors in geology has succeeded in introducing so many new ideas into the science as the first great observer. For several centuries most of his successors in geology remained far behind him in creative genius, and so there is little progress worth while noting in the knowledge of the method of earth formation, until almost the beginning of the nineteenth century, though his little book was written in 1668 and 1669.
Leibnitz regretted very much that Stensen did not complete his work on geology as he originally intended. Had he succeeded in gathering together all of his original observations, illustrated by the material he had collected, his work would have had much greater effect. As it was, the golden truth which he had expressed in such [{160}] few words, without being able always to state just how he had come to his conclusions, was only of avail to science in a limited way. Men had to repeat his observations long years afterwards in order to realize the truth of what he had laid down. Leibnitz considered that it took more than a century for geological science to reach the point at which it had been left by Steno's work, and which he had reached at a single bound. There is scarcely a single modern geologist interested at all in the history of the science who has not paid a worthy tribute to Steno's great basic discoveries in the science. It was not a matter for surprise, then, that the International Congress of Geologists which met at Bologna in 1881 assembled at his tomb in Florence in order to do him honor, after the regular sessions of the Congress had closed. They erected to his memory a tablet with the following scription:
"Nicolae Stenonis imaginem vides hospes quam aere collato docti amplius mille ex universo terrarum orbe insculpendam curarunt in memoriam ejus diei IV cal. Octobr. an. MDCCCLXXXI quo geologi post conventum Bononiae habitum praeside Joanne Capellinio equite hue peregrinati sunt atque adstantibus legatis flor Municipii et R. Instituti Altiorum doctrinarum cineres viri inter geologos et anatomicos praestantissimi in hujus templi hypogaeo laurea corona honoris gratique animi ergo honestaverunt." [Footnote 12]
[Footnote 12: You behold here, traveller, the bust of Nicholas Steno as it was set up by more than a thousand scientists from all over the world, as a memorial to him, on the fourth of the Kalends of October, 1881. The geologists of the world, after their meeting in Bologna, under the presidency of Count John Capellini, made a pilgrimage to his tomb, and in the presence of the chosen representatives of the municipality, and of the learned professors of the University, honored the mortal ashes of this man, illustrious among geologists and anatomists.]
Stensen's work brought him in contact with some of the distinguished men of the seventeenth century, all of whom learned to appreciate his breadth of intelligence and acuity of judgment. We have already mentioned his epistolary relation with Spinoza, and have said something about the controversy with Leibnitz, into which, in spite of his disinclination to controversy generally, he was drawn by the circumstances of the time and the solicitation of friends. Another great thinker of the century with whom he was brought into intimate relationship was Des Cartes, the distinguished philosopher. In fact, Des Cartes's system of thought influenced Stensen not a little, and he felt, when describing the function of muscles in the human body, and especially when he demonstrated that the heart was a muscle, that the mechanical notions he was thus introducing into anatomy were likely to prove confirmatory of Des Cartes's philosophic speculations. Almost more than any other, Stensen was the father of many ideas that have since become common, with regard to the physics of the human body and its qualities as a machine.
With his breadth of view, from familiarity [{162}] with the progress of science generally in his time, Steno's discussions of the reason for the lack of exact knowledge and for the prevalence of error, in spite of enthusiastic investigation, are worth while appreciating. He considered that the reason why so many portions of natural science are still in doubt is that in the investigation of natural objects no careful distinction is made between what is known to a certainty and what is known only with a certain amount of assurance. He discusses the question of deductive and inductive science, and considers that even those who depend on experience will not infrequently be found in error, because their conclusions are wider than their premises, and because it only too often happens that they admit principles as true for which they have no sure evidence. Stensen considered it important, therefore, not to hurry on in the explanation of things, but, so far as possible, to cling to old-time principles that had been universally accepted, since nearly always these would be found to contain fruitful germs of truth.