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single set of teeth, and the Diphyodonts1 or those that generate two sets of teeth. But this dental character is not so associated with other organic characters as to indicate natural or equivalent sub-classes.
In the Mammalian orders with two sets of teeth, these organs acquire individual characters, receive special denominations, and can be determinated from species to species. This differentiation of the teeth is significative of the high grade of organization of the animals manifesting it.
Originally, indeed, the names 'incisors,' 'canines,' and 'molars,' were given to the teeth, in Man and certain Mammals, as in Reptiles and Fishes, in reference merely to the shape and offices indicated by those names; but they are now used as arbitrary signs, in a more fixed and determinate sense. In some Carnivora, e. g., the front teeth have broad tuberculate summits adapted for nipping and bruising, while the principal back-teeth are shaped for cutting and work upon each other like the blades of scissors. The front-teeth in the Elephant project from the upper jaw, in the form, size and direction of long pointed horns. Indeed, shape and size are the least constant of dental characters in the Mammalia; and the homologous teeth are determined, like other parts, by their relative position, by their connexions, and by their development.
Those teeth which are implanted in the premaxillary bones, and in the corresponding part of the lower jaw, are called 'incisors' (fig. 2, i), whatever be their shape or size. The tooth in the maxillary bone, which is situated at or near to the suture with the premaxillary, is the 'canine,' as is also that tooth in the lower jaw (ib. c), which, in opposing it, passes in front of the upper one's crown when the mouth is closed. The other teeth of the first set are the 'deciduous molars'(d. 1—3); the teeth which displace and succeed them vertically are the 'premolars' (p. 1—3) ; the more posterior
1 51s, twice; <piu and 6&ois. See Philosophical Transactions, 1850, p. 493.
teeth, which are not displaced by vertical successors, are the 'molars' properly so called (m. 1—4).
Lower Jaw of a young Opossum (Didelphys).
I have been led, chiefly by the state of the dentition in most of the early forms of both carnivorous and herbivorous Mammalia, which flourished during the eocene tertiary periods, to regard 3 incisors, 1 canine, and 7 succeeding teeth, on each side of both jaws, as the type formula of diphyodont dentition.
Three of the seven teeth may be 'premolars' (fig. 2, p. 1—3), and four may be true 'molars' (ib. m. 1—4); or there may be four premolars (fig. 3, p. 1—4), and three true molars (ib. m. 1—3). This difference forms a character of an
ordinal group in the mammalian classThe essential nature of the distinction is as follows: true molars (ib. m.) are a backward continuation of the first series of teeth (ib. d.); they are developed in the same primary groove of the foetal gum; they are 'permanent' because they are not pushed out by the successional teeth (ib.p.), called 'dents de remplacement' by Cuvier. Seven teeth developed in the primary groove is,
1 Outlines of a Classification of the Mammalia, Trans. Zool. Soc. Vol. n. P- 33° (i839)
therefore, the typical number of first teeth, beyond the canines. If, as in Didelphys (fig. 2), the anterior three develope toothgerms which come to perfection in a 'secondary groove,' there are then 3 deciduous teeth, 3 premolars, and 4 true molars: if, as in Sus, fig. 3, the anterior four of the 'primary' teeth develope tooth-germs, which grow in a secondary groove, there are then 4 deciduous teeth, 4 premolars, and 3 true molars. The first true molar of the marsupial (fig. 2, m. 1, d. 4), is thus seen to be the homologue of the last milk-molar of the placental (fig. 3, d. 4).
The Hog, the Mole, the Gymnure and the Opossum, are among the few existing quadrupeds which retain the typical number and kinds of teeth. In a young Hog of ten months (fig. 3), the first premolar, p. 1, and the first molar, m. 1, are in place and use together with the three deciduous molars, d. 2, d. 3, and d. 4; the second molar, m. 2, has just begun to cut the gum; p. 2, p. 3, and^?. 4, together with m. 3, are more or less incomplete, and will be found concealed in their closed alveoli1.
The last deciduous molar, dA, has the same relative superiority of size to d. 3 and d. 2, which m. 3 bears to m. 2 and m. 1; and the crowns of p. 3 and p. 4 are of a more simple form than those of the milk-teeth, which they are destined to succeed. When the milk-teeth are shed, and the permanent ones are all in place, their kinds are indicated, in the genus Sus, by the following formula:—
. 3—3 1—1 4—4 3—3 «-3=3> CT=I' P'i=Z> W"3=3 = M' which signifies that there are on each side of both upper and lower jaws 3 incisors, 1 canine, 4 premolars, and 3 molars, making in all 44 teeth, each tooth being distinguished by its appropriate symbol, viz. p. 1 topA,m.\ to m.3. This number of teeth is never surpassed in the placental diphyodont series.
1 I recommend this easily acquired 'subject' to the young zoologist for a demonstration of the most instructive peculiarities of the mammalian dentition. He will see that the premolars must displace deciduous molars in order to rise into place: the molars have no such relations.
When the premolars and the molars are below this typical number, the absent teeth are missing from the back part of the molar series, and usually from the fore part of the premolar series. The most constant teeth are the fourth premolar and the first true molar. These being known by their order and mode of development, the homologies of the remaining molars and premolars are determined by counting the molars from before backwards, e. g. 'one,' 'two,' 'three,' and the premolars from behind forwards, 'four,' 'three,' 'two,' 'one.' The incisors are counted from the median line, commonly the foremost part, of both upper and lower jaws, outwards and backwards. The first incisor of the right side is the homotype, transversely, of the contiguous incisor of the left side in the same jaw, and vertically, of its opposing tooth in the opposite jaw; and so with regard to the canines, premolars, and molars; just as the right arm is the homotype of the left arm in its own segment, and also of the right leg of a succeeding segment. It suffices, therefore, to reckon and name the teeth of one side of either jaw in a species with the typical number and kinds of teeth, e. g. the first, second, and third incisors,—the first, second, third, and fourth premolars, —the first, second, and third molars; and of one side of both jaws in any case.
I have been induced to dwell thus long on the dental characters of the class Mammalia, because they have not been rightly defined in any systematic or elementary work on zoology, although an accurate formula and notation of the teeth are of more use and value in characterizing genera in this than in any other class of animals.
Mammals may be surpassed in the rapidity with which the blood circulates, in the extent and completeness of the respiratory processes, in bodily temperature, in the concomitant vigour of the muscular actions; all which superiorities, in Birds, for example, result in those marvellous powers of flight with which the feathered class is privileged. But in their psychical phenomena the Mammalia, as a class, excel all other animals. Let me exemplify this by reference to the reproductive economy in the vertebrate series.
The instinctive sense of dependence upon another, manifested by the impulse to seek out a mate,—which impulse, even in fishes, is sometimes so irresistible that they throw themselves on shore in the pursuit,—this first step in the supercession of the lower and more general law of individualor self-preservation, although not first introduced at the vertebrate stage of the animal series, is never departed from after that stage has been gained. To this sexual relation is next added a self-sacrificing impulse of a higher kind, viz. the parental instinct. As we rise in the survey of vertebrate phenomena, we see the entire devotion of self to offspring in the patient incubation of the bird, in the unwearied exertions of the Swift or the Hawk to obtain food for their callow brood when hatched; in the bold demonstration which the Hen, at other times so timid, will make to repel threatened attacks against her cowering young.
Still closer becomes the link between the parent and offspring in the Mammalian class, by the substitution, for the exclusion of a passive irresponsive ovum, of the birth of a living young, making instinctive irresistible appeal, as soon as born, to maternal sympathy; deriving nutriment immediately from the mother's body, and both giving and receiving pleasure by that act.
These beautiful foreshadowings of higher attributes are, however, transitory in the brute creation, and the relations cease, as soon as the young quadruped can provide for itself. Preservation of offspring has been superinduced on self-preservation, but there is as yet no self-improvement: this is the peculiar attribute of mankind. The human species is characterised by the prolonged dependence of a slowly maturing offspring on parental cares and affections, in which are laid the foundations of the social system, and time given for instilling those principles on which Man's best wisdom and truest happiness are based, and by which he is prepared for another and