Farther discoveries of fossil remains are also needed to make known the antetypes, in which varieties, analogous to the observed ones in existing species, might have occurred, seriatim, so as to give rise ultimately to such extreme forms as the Giraffe.
This application of paheontology has always been felt by myself to be so important that I have never omitted a proper opportunity for impressing the results of observations showing the "more generalized structures" of extinct, as compared with the " more specialized forms" of recent animals.
But observation of the effects of any of the above hypo thetical transmuting influences in changing any known species into another has not yet been recorded. And past expe rience of the chance aims of human fancy, unchecked and unguided by observed facts, shows how widely they have ever glanced away from the gold centre of truth.
The principles, based on rigorous and extensive observa tion of facts, which have thus been inductively established, and have tended to impress upon the minds of the closest reasoners in Biology a conviction of a continuously operative secondary creational law, are the following :—the law of irre lative or vegetative repetition : the law of unity of plan or rela tions to an archetype : the phenomena of parthenogenesis : the progressive departure from general type as exemplified in the series of species from their first introduction to the present time.
The Table (fig. 142) expresses the sum of the observa tions at the present date, on the succession, appearance, and geological relations of the several orders of the Mammalian class.
The earliest evidences are of small species, which, when ever they have presented grounds for ordinal determination, have proved to belong to the low organized Marsupialia. The doubt, when it has existed, lies between this and the Insecti vorous order, also low in the class according to cerebral One example only, from Stonesfield oolite, the Stereognathus, may prove to be a minute Ungulate, as is indi cated by the note of interrogation under Perissodactyla. The similar mark, under Cetacea, refers to the fossil, probably washed out of an Upper Oolitic bed, referred to at p. 321. The Marsupialia recur, under distinct generic forms, in the eocene strata, and, according to actual knowledge, present their fullest development in pliocene and modern times, more especially in Australia. The orders Bruta, Perissodactyla, and Carnivora,
have become reduced in numbers ; the Proboscidia still more so ; the representatives of the singular group Toxodontia have wholly disappeared.
The sum of the evidence which has been obtained seems to prove that the successive extinction of Microlestes, Arnphitheria, Spalacotheria, Tricot:04ns, and other mesozoic forms of mammals, has been followed by the introduction of much more numerous, varied, and higher-organized forms of the class, during the tertiary periods.
It may be, however, objected that negative evidence cannot satisfactorily establish the proposition that the mammalian class is of late introduction, nor prevent the conjecture that it may have been as richly represented in primary and more ancient secondary as in tertiary times, could we but get remains of the terrestrial fauna of the continents. To this objection it may be replied: in the palwozoic strata, which, from their extent and depth, indicate, in the earth's existence as a seat of organic life, a period as prolonged as that which has followed their deposition, no trace of mammals has been observed. Were mammals peculiar to dry land, such nega tive evidence would weigh less in producing conviction of their non-existence during the Silurian and Devonian (eons, because the explored parts of such strata have been deposited from an ocean, and the chance of finding a terrestrial and air breathing creature's remains in oceanic deposits is very remote. But in the present state of the warm-blooded, air-breathing, viviparous class, no genera and species are represented by such numerous and widely dispersed individuals, as those of the order Cetacea, which, under the guise of fishes, dwell, and can only live, in the ocean.
In all Cetacea the skeleton is well ossified, and the vertebras are very numerous : the smallest Cetacean would be deemed large amongst land-mammals; the largest surpass in bulk any creatures of which we have yet gained cognizance: the hugest ichthyosaur, iguanodon, megalosaur, mammoth, or megathere, is a dwarf in comparison with the modern whale of a hundred feet in length.