The chemical nature of the hardening particles, and of the soft basis of bone, is exemplified in the subjoined table, includ ing a species of each of the four classes of Vertebrata The most common change which bones first undergo is the loss of more or less of their original soft and soluble basis. This effect of long interment is readily tested by applying the specimen to the tongue, when the affinity of the pores of the earthy constituent, after having lost the gelatine for fluid, is so great, that the specimen adheres to the tongue like a piece of dry chalk. Bones and teeth in this state quickly absorb a solution of gelatine, and thus their original tenacity may be restored.* Petrified fossils need no such treatment ; they are usually harder and more durable than the original bone itself.
The interpretation of such fossil remains requires a com parison of them with the corresponding parts of animals now living, or of previously determined extinct species. In the case of the vertebrate animals, such comparison is limited to the osseous and dental systems. The interpretation of a vertebrate fossil, therefore, presupposes a knowledge of the various modi fications of the skeleton and teeth of the existing vertebrate animals ; and the more extensive and precise such knowledge may be, the more successful will be the efforts, and the more exact the conclusions, of the interpreter.
The determination of the remains of quadrupeds is beset, as Cuvier truly remarks, with more difficulties than that of other organic fossils. Shells are usually found entire, and with all the characters by which they may be compared with their analogues in the museums, or with figures in the illus trated books, of naturalists. Fishes frequently present their skeleton or their scaly covering more or less entire, from which may be gathered the general form of their body, and frequently both the generic and specific characters which are derived from such internal or external hard parts. But the entire skeleton of a fossil quadruped is rarely found, and when it occurs, it gives little or no information as to the hair, the fur, or the colour of the species. Portions of the skeleton with the bones dislocated, or scattered pell-mell—detached bones and teeth, or their fragments merely—such are the conditions in which the petrified remains of the mammalian class most commonly present themselves in the strata in which they occur.
Prior to the time of Cuvier but little progress had been made in the interpretation of such fragmentary remains. The striking success which attended the application of the great comparative anatomist's science to this previously neglected field of study, was referred by Cuvier to principles in the organization of animal bodies, which he termed the " Correla tion of Forms and Structures," and the " Subordination of Organs"—principles which his clear-thinking biographer, M. Flourens,* in common with most contemporary philosophers, has regarded as the most effective and successful instrument in the restoration of extinct animals. They will be exempli fied in the course of the present section of this work.
A terminal phalanx modified to fit a hoof may give, as Cuvier declared, the modifications of all the bones of the fore limb that relate to the absence of a rotation of the fore leg, and all the modifications of the jaw and skull that relate to the mastication of food by broad-crowned complex molars.
But there are certain associated structures for the coinci dence of which the physiological law is unknown. " I doubt," writes Cuvier, " whether I should have ever divined, if observa tion had not taught it me, that the ruminant hoofed beasts should all have the cloven foot, and be the only beasts with horns on the frontal We know as little why horns should be in one or two pairs on the frontal bone of those Ungulates only which have hoofs in one or two pairs ; whilst in the horned Ungulates with three hoofs, there should be either one horn, or two horns placed one behind the other in the middle line of the skull ; or why the Ungulates with one or three hoofs on the hind foot should have three trochanters on the femur, whilst those with two or four hoofs on the hind foot should have only two trochanters.
" However," continues Cuvier, " since these relations are constant., they must have a sufficing cause ; but as we are ignorant of it, we must supply the want of the theory by means of observation.* This, if adequately pursued, will serve to establish empirical laws almost as sure in their application as rational ones." " That there are secret reasons for all these relations, observation may convince us independently of general philosophy." " The constancy between such a form of such organ, and such another form of another organ, is not merely specific, but one of class, with a corresponding gradation in the development of the two organs."t
"For example, the dentary system of non-ruminant Ungu lates is generally more perfect than that of the Bisulcates ; inasmuch as the former have almost always both incisors and canines in the upper as well as the lower jaw ; the structure of their feet is in general more complex, inasmuch as they have more digits, or hoofs less completely enveloping the phalanges, or more bones distinct in the metacarpus and metatarsus, or more numerous tarsal bones ; or a more distinct and better developed fibula ; or a concomitance of all these modifications. It is impossible to assign a reason for these relations ; but, in proof that it is not an affair of chance, we find that whenever a bisulcate animal shows in its dentition any tendency to approach the non-ruminant Ungulates, it also manifests a similar tendency in the conformation of its feet. Thus the camels, which have canines and two or four incisors in the upper jaw, have an additional bone in the tarsus, resulting from the scaphoid not being confluent with the cuboid ; and the small hoofs have correspondingly small phalanges. The musk deer, which have long upper canines, have the fibula co-exten sive with the tibia, whilst the other ruminants have a mere rudiment of fibula articulated to the lower end of the tibia." " There is then a constant harmony between two organs to all appearance quite strangers to each other, and the gradations of their forms correspond uninterruptedly even in the cases where one can render no reason for such relations." " But in thus availing ourselves of the method of observation as a supple mentary instrument when theory abandons us, we arrive at astonishing details. The smallest articular surface (faeette) of a bone, the smallest process, presents a determinate character relating to the class, to the order, to the genus, and to the species to which they belong ; so that whoever possesses merely the well-preserved extremity of a bone, may, with applica tion, aided by a little tact (adresse) in discerning analogies, and by sufficient comparison, determine all these things as surely as if he possessed the entire animal."* There have been, of course, instances, and will be, where for want of the " efficacious comparison," and the " tact in dis cerning likeness," such results have not rewarded the endea vours of the palaeontologist ; and these shortcomings, and the mistakes sometimes made, even by Cuvier himself, have been cast in the teeth of his disciples, as arguments against the principles by which they believed themselves guided in their endeavours to complete the glorious edifice of which their master laid the foundations.
The writer has, therefore, quoted from the well-known " Preliminary Discourse" to Cuvier's great work on Fossil Remains, with a view to neutralize the efforts of statements reiterated in apparent ignorance of the clear and explicit man ner in which Cuvier there defines the limits within which the law of correlation of animal structures may be successfully applied, and indicates the instances in which—the physio logical condition being unknown, and the coincident structures being understood empirically—careful observation and rigor ous comparison must supply the place of the physiologically understood law.
Those who deny the existence of design in the construction of any part of an organized body, and who protest against the deduction of a purpose from the valves of the veins or the lens of the eye-ball, repudiate the reasoning which the palxonto logist carries out from the hoof to the grinder, or from the carnassial molar to the retractile claw, through the guidance of the principle of a pre-ordained mutual adaptation of such parts ; but such minds are not nor have been, those who have contributed to the real advancement of physiology or palaeontology.
By reference to the "Table of Strata" (fig. 1), it will be seen that the earliest evidence of a vertebrate animal is of the cold-blooded water-breathing class in the upper Silurian period. Next follows that of a cold-blooded but air-breath ing vertebrate, under the batrachian grade, in the carbonifer ous period. The warm-blooded air-breathing classes are first indicated, as birds, by footmarks in a sandstone of probably triassic but not older age ; and, as mammals, by fossil teeth from bone-beds of the upper triassic system in Wirtemberg, and of the same age near Frome, Somersetahire. Mam malian remains have also been found in a coal-field in North Carolina, which may be earlier, but cannot be later, than the liar formation.