On the subject of vegetables we need add nothing to the preceding remarks, because nothing can here be prov ed by a mere examination of facts. It must also be ap parent that we are unable to pronounce decidedly on the extinction of any marine animals, for the reasons formerly assigned, namely, our uncertainty respecting the number and natures of those now existing. It is nevertheless probable that the numerous researches made in the de partment of conchology in so many parts of the world, would have discovered more correspondence, had many not been destroyed. The ammonites are among those which naturalists have been least successful in discovering, as are the orthoceratites,nautilites,belemnites, and others; but whatever we may imagine on this subject, whether with regard to extinct genera or species, the proofs will probably never extend beyond a high degree of probabi lity. We need not therefore prolong remarks which con sist of nothing but conjecture; although it is not useless to caution our readers against drawing general and posi tive conclusions from partial and negative evidence.
The extinction of species, and even of genera, among the quadrupeds, or larger land animals, found in the upper strata or in the alluvia, is almost matter of demonstration, though that question, as it relates to all the buried indi viduals, of which the living parallels have not been found, can never be settled until we have explored all the yet unknown regions of the globe. In the mean time, as far as our knowledge of the living creation at present extends, the following statement on this subject has been made by Cuvier, to whom we are indebted for the greater part of our knowledge respecting it. Of seventy-eight species which he examined, there are forty-nine unknown, and sixteen or eighteen doubtful, as we already remarked in speaking of those of which the species still exist. These forty-nine species comprise seven new genera, which in elude twenty-seven of the number, and the remaining twenty-two belong to sixteen genera still existing. The whole number of genera, therefore, under which these forty-nine species are included, is thirty-six.
It has been objected to Cuvier's conclusions, as he says, that the present existing races of land quadrupeds may be only modifications or varieties of the fossil ones, and that these changes may have been the consequence of alterations in the climate and other local circumstances operating through a long series of ages. To this the answer is easy, independently of the absolute assumption which is here involved, and it is, that there are no traces to be found of such a gradual modification. Nothing interme diate is found between the PaIxotherium, for example, and any animal of the present world ; so that we are entitled to conclude, that the ancient species were fully as steady and definite as the modern ones.
The question of the range to which varieties may extend in animals, forms part of the argument on this subject. His definition of a species is, that " it compre
hends all the individuals which descend from each other, or from a common parentage, together with those which resemble them as much as they do each other." It is only, therefore, by comparing the ancestors with their descendants, if we can determine these, that we can learn what the extent of the varieties may be. Taking this rule as a guide, it will be found that the differences which constitute varieties are limited to a particular set of cir cumstances. Among these the foremost are colour, which depends on light and situation; the hair, as to length or quantity, which depends on the temperature; and the size, which is modified by the abundance or deficiency of food. In the wild kinds, or individuals, however, all these pos sible variations are very much limited by the natural habits of the animal, which does not willingly change its climate, and is commonly steadily supplied with the same kind and quantity of food. Even in the wolf and fox, which range from the torrid to the frigid zone, there is little difference in the varieties beyond the colour, fine ness, and quantity of the hair. The more confined wild tribes vary still less; so that between the hyxna of Persia and that of Morocco there is no other difference than that of a thicker or thinner mane.
In the wild herbivorous animals the effects of climate are more sensible, because the quantity and quality of the food are in a good measure regulated by that important circumstance. It is hence that the elephants of different countries vary so much, even at small distances; and that is very often sensible in the length of the tusks. The same occurs in the deer tribe, and in the ox; yet in none of these, even when the variations are apparently greatest, are there any difference in the form or number of the bones or the teeth, or in any of the essential parts of the animal structure.
Against such variations of form as might arise from intermixture of breeds, nature has notoriously provided, by rendering the greater number impossible, and by mak ing the produce of those that are so sterile. Such pro ducts are also unnatural; they do not occur in a state of nature, but are the consequence of domestication and contrivance on our part. Thus also domestication, in a general manner, produces all the chief varieties of animals; although, even in these cases, it is seldom able to do more than change the size of the creature, the colour, the quantity of fur, and the forms of the horns. Every thing important remains the same. Thus, in the sheep, as well as in the ox, in the horse, and in the stag, the changes thus produced are in general quite superficial, or occur in unimportant parts of the structure; the greatest varia Lion ever produced being some difference in the propor tional lengths of the bones and teeth ; although in the hog, a solitary instance is afforded of a change so great as to have united the cloven hoof into one.