But it is necessary to go into the subject at some length on account of the great importance which has been laid upon the teeth in deciding questions of relationship; moreover, largely no doubt on account of their hardness and imperishability, our knowledge of certain extinct forms of Mammalia is entirely based upon a few scattered teeth; while of some others, notably of the Triassic and Jurassic genera, there is not a great deal of evidence except that which is furnished by the teeth. Indeed the important place which odontography holds in comparative anatomy is from many points of view to be regretted, though inevitable. "In hardly any other system of organs of vertebrated animals," remarks Dr. Leche, "is there so much danger of confounding the results of convergence of development with true homologies, for scarcely any other set of organs is less conservative and more completely subservient to the lightest impulse from without." Affinities as indicated by the teeth are sometimes in direct contradiction to those afforded by other organs; or, as in the case of the simple Toothed Whales, no evidence of any kind is forthcoming. Dr. Leche has pointed out that, judged merely from its teeth, _Arctictis_ would be referred to the Raccoons, though it is really a Viverrid; while _Bassariscus_, which Sir W. Flower showed to be a Raccoon, is in its teeth a Viverrid. Mr. Bateson has been obliged to hamper the subject with another difficulty. In dealing with the variations of teeth,[25] Mr. Bateson has brought together an immense number of facts, which tend to prove that the variability of these structures is much greater than had been previously recognised; that this variability is often symmetrical; and that in some animals, as in "_Canis cancrivorus_, a South American fox, the majority showed some abnormality." When we learn from Mr. Bateson that "of _Felis fontanieri_, an aberrant leopard, two skulls only are known, both showing dental abnormalities," it seems dangerous to rear too lofty a superstructure upon a single fossil jaw. It must be noted too that, {47} contrary to the prevailing superstition, it is not domestic animals which show the greatest amount of tooth variation. As to special homologies between tooth and tooth, with which we shall deal on a later page, Mr. Bateson has urged almost insuperable difficulties. [Illustration] FIG. 34.--Skull of Dasyurus (lateral view). _al.sph_, Alisphenoid; _ang_, angular process of mandible; _fr_, frontal; _ju_, jugal; _lcr_, lachrymal; _max_, maxilla; _nas_, nasal; _oc.cond_, occipital condyle; _par_, parietal; _par.oc_, paroccipital process; _p.max_, premaxilla; _s.oc_, supraoccipital; _sq_, squamosal; _sq_', zygomatic process of squamosal. (From Parker and Haswell's _Zoology_.) [Illustration] FIG. 35.--Upper and lower teeth of one side of the mouth of a Dolphin (_Lagenorhynchus_), illustrating the homodont type of dentition in a mammal. (After Flower and Lydekker.)