IN THE LIMBS OF MAMMALIA. 51 



These are, 1st, the law of Parallel Relations ; and, 2d, the teleological rising above or 

 sinking below their morphological level, of certain groups or species or individuals, whereby 

 they seem to be of a higher or lower grade than the rest of the group of which they are 

 generally the extreme aberrant forms. 



Illustrations of either of these laws are almost superfluous; of the former many will have 

 occurred to the naturalist who observes similar functions exercised by animals belonging to 

 different groups, or even types: — the aerial bird and butterfly; the heavy, graminivorous 

 cattle among Mammalia, and the phytophagous Scarabei among Coleoptera ; the monkey 

 and the parrot; the whole type of Articulates, and the vertebrate class Aves; the type of 

 Mollusks, and the vertebrate class Reptilia ; the three classes of Articulates, with the three 

 orders of its highest class, Insecta ; and, finally, the striking parallelism between the orders 

 of the two groups of Mammalia, called by Dana Megasthenes and Microsthenes, (American 

 Journal of Science and Art, vol. xxxv. p. 70,) with the less evident one between the Altrices 

 and the Precoces among birds. Between all these pairs of groups is so evident a similarity 

 as to have suggested the term "Parallel Relation;" but it is to be observed that the rela- 

 tion is one of analogy, not homology ; that the differences are morphological, and the resem- 

 blances are comparatively teleological, while between component parts of the same group 

 the resemblances are morphological and the differences teleological. 



Many insects are physiologically more highly organized than the lowest fishes, and the 

 eagle seems a creature vastly superior to the whale; but in each case the groups to which, 

 according to their essential structure, the insect and the eagle belong, are, as groups, on a 

 plane below the fishes and the mammals. The two relations are commonly expressed by 

 representing the groups by parallel vertical lines ; there may be such morphological differ- 

 ences between the groups as to clearly indicate which are higher and which lower, but the 

 lines may be overlapped, to show that the lowest in one group is teleologically inferior to 

 the highest of the next group, though, as said above, there would be no doubt concerning 

 the groups taken as wholes. 



There is not, at least among the higher groups, any such lineal shading off" into each 

 other as to afford any support to the idea of a regular, uninterrupted succession of organic 

 forms, whether zoological or genealogical. Nor does the present state of Paleontology fur- 

 nish the disciples of Darwin much assistance in this respect. 



Position may determine a morphology in addition to that dependent upon structure, and 

 nowhere is this more clear than with the teeth of Mammalia. Professor Owen, in his 

 Odontography, has shown that every classification of these organs based upon their form, 

 and thus upon their special masticatory function, utterly fails in precision on general appli- 

 cation, and that 'the position of the teeth in the jaws is the only safe guide to their arrange- 

 ment. In this case, it so happens that the teeth were originally named, from their shape 

 and function, incisors, canines, and molars ; and this is the order in which they stand in the 

 jaws from before backward. But, while this would answer very well in designating the 

 corresponding teeth in two animals having the same number, and where the variations in 

 form were slight, it utterly failed, even in the hands of Cuvier, accurately to determine 

 such correspondence when applied to the whole range of the mammalian series. 



Without entering into details, which are given in abundance in the Odontograplvy, it 

 may be said that the teeth collectively are distinguished from all other parts and organs, 

 hard or soft, by a peculiar structure or morphology of their own ; but that, to ascertain the 

 limits of the several groups of teeth in the jaws of a single species, or to point out corre- 

 sponding or homologous teeth in animals having a different number, their position in the 

 jaws is the only safe standard, this constituting a minor morphology. 



