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Zootoka, or air-breathing vivipara, are divided according to the nature of their limbs into three sections:—1st, Dipoda; 2nd, Tetrapoda; 3rd, Apoda. The first comprised the biped human race, the second the hairy quadrupeds, the third the whale-tribe, in which the limbs answering to the legs of man are wanting.

The second of these divisions, which includes the great majority of mammals, and is commonly regarded as the class itself, Aristotle subdivides into two great groups, according to the modification of the extremities. In the first group the foot is multifid, and a part of the digit—finger or toe—is left free for the exercise of the faculty of touch, the hard nail or claw being placed upon one side only; in the second group the digits are inclosed in hoofs: these groups are recognised in modern Zoology as the Unguiculata and Ungulata.

Aristotle, in the generalised expressions of his observations on the various conditions of the teeth, has indicated subdivisions of the Unguiculata according to characters of the dental system. One subdivision includes those quadrupeds which have the front teeth trenchant, and the back teeth flattened, viz. the Pithecoida or Ape-tribe. Another subdivision includes the quadrupeds with diversified acuminated front teeth and interlocking serrated back teeth, viz. the Karcharodonta, or Carnivora; whilst the animals now known as 'Rodents' are indicated by a negative dental character.

With respect to the hoofed or Ungulate quadrupeds Aristotle in his generalisations on the organs of progressive motion divides them into Dischidce, or bisulcate quadrupeds, and Aschidce, or solidungulates, e. g. the horse and ass.

The term Anepallacta, by which Aristotle signified the animals in which the upper and lower teeth do not interlock, is applicable to the herbivorous quadrupeds generally; in which the Amphodonta, or those with teeth in both jaws, e. g. the horse, are distinguished by him from those in which the front teeth are wanting in the upper jaw, e. g. the ox.

The bats were rightly recognised as true Zootoka, and the genus was defined as Dermaptera.

The apodal Vivipara, which form the third of Aristotle's more comprehensive groups, embraces the Ketode, now called Cetacea, and affords, by its position and co-ordinates in the great philosopher's zoological system, one of the most striking examples of his sagacity and research. In generalising, however, on modes of reproduction Aristotle includes certain sharks with the cetaceans, distinguishing the former by their gills, the latter by their blow-hole.

I ought, also, to remark that, although Aristotle has exemplified groups of animals which agree with many of the modern Classes, Orders, and Genera, their relative value is not so defined1; and his, in most respects, natural, assemblages would have commanded greater attention and been earlier and more generally recognised as the basis of later systems, had its immortal author more technically expressed an appreciation of the law of the subordination of characters; but Aristotle applies to each of his groups the same denomination, viz. yevos, genus; distinguishing, however, in some cases, the greater from the less.

Centuries elapsed ere any advance was made in the science of Zoology as it was bequeathed to the intellectual world by the mind of Aristotle. Of no other branch of human knowledge does the history so strongly exemplify the fearful phenomenon of the arrest of intellectual progress, resulting in the 'dark ages.' The well-lit torch which should have guided to further explorations of the mighty maze of animated nature was suffered to fall from the master-hand, and left to grow dim and smoulder through many generations ere it was resumed, fanned anew into brightness, and a clear view regained both of the extent of ancient discovery and of the right course to be pursued by modern research.

1 See the just and discerning remarks on this subject by Dr Whewell, in his admirable History of the Inductive Sciences, 3rd ed., Vol. III. p. 289.

B2

To John Ray, an ornament of this University, I would ascribe the merit of proposing a classification of the Zootoka, which first claims attention as in any respect an advance upon that taught by the Father of Natural History. It is given in a tabular form in Ray's Synopsis Methodica Animalium Quadrupedum, and is as follows:—(See p. 5).

In this Table the principle of the subordination of characters, or of their different values as applicable to groups of different degrees of generalisation, is clearly exemplified; and herein perhaps is its chief value. But, in the exclusion of the Dipoda and Apoda of Aristotle, Ray manifests a less philosophical appreciation of the extent and essential nature of the class Zootoka than his great predecessor. He is also inferior in the discernment of the real significance of certain modifications of zoological characters. Aristotle was not deceived either by the claw-like shape of the hoofs of the camel, or by the degree of subdivision of those of the elephant; he knew that both quadrupeds were, nevertheless, essentially Ungulate1.

Linnaeus first definitely and formally restored the great natural class I am now treating of to its Aristotelian integrity; and, applying to it that happy instinct of discernment of significant outward characters which had enabled him to effect so much for the sister Science of Botany, he proposed for it the name Mammalia.

The active cultivation of the science of observation stimulated by Ray, Linnaeus and Buffon, had brought to light instances, e. g. in certain lizards, of viviparous quadrupeds which differ in structures of classific importance from the Zootoka tetrapoda of Aristotle. Certain forms of true fishes were now known to bring forth their young alive, as well as the fish-like Ketode. The term Zootoka ceased to be applicable, exclusively, to the class of which Aristotle had sketched out the bounds; and Naturalists gladly accepted and have since retained the neat and appropriate and truly distinctive

1 'Ko2 ivrl ov6xwv XV^to *bcel"'

A TABLE OF VIVIPAROUS FOUR-FOOTED ANIMALS.

Viviparous hairy animals or quadrupeds are,— f Ungulate, and these either

f Solidipedous, as the E,,, Ass, Hhm. Bisulcate, which are

"Ruminants with horns, that are

( Persistent, as in the Ox, Ee Ee,
or <. or

I Deciduous, as in the He
Not Ruminants, as the ,e
Quadrisulcate, as the Ehh Eheeorhs.
(_ Unguiculate, whose feet are either

(Bifid, as in the Caoree, or

XMultifid, which are .

i With digits adhering together, and covered with a common integument, so that the extremities alone are jy,
} visible at the margin of the foot, and are covered with obtuse nails, as in the Eeeeee
'With digits in some measure distinct and separable from each other, the nails being {Depressed, as in Heh
or
Compressed, where the incisor teeth are

f Many, in which group all the animals are carnivorous and rapacious, or at least insectivorous, or subsist on insects with vegetable matter:
- The larger ones with the

{Muzzle short, and head rounded, as the Feline tribe;or
with the
Muzzle long, as the Canine tribe;
The smaller ones with a long slender body, and short extremities, as the
Weasel or Vermine1 tribe;
[_ Two very large, of which tribe all the species are phytivorous, as the Ee.

l Genus Vermineum, from their worm"like form. term proposed by Linnaeus,—the term which was suggested by the outward and visible part of that apparatus by which the warm-blooded viviparous animals exclusively nourish their new-born young1.

Linnaeus, like Ray, founds his primary divisions of the class Mammalia on the locomotive organs; but his secondary divisions or orders are taken chiefly from modifications of the dentary system. The following is an abridged scheme of his arrangement8:—

IFront teeth, none in either jaw . . . Bruta. Front teeth, cutters i, laniaries o . . . Glires. Front teeth, cutters 4, laniaries 1 . . Primates.

Front teeth, piercers (6, i, 10), laniaries 1 Feb.«.

S j p , , ( Front teeth, in both upper and lower jaw. BELLU.fi.

J unOwuue \ Front teeth, none in the upper jaw . . Pecoba.

S L Mutkate Teeth variable Cete.

On comparing the three preceding systems, it will be found that the most important errors of arrangement have been committed, not by Aristotle, but by the modern naturalists. Both Ray and Linnaeus have mistaken the character of the horny parts enveloping the toes of the elephant, which do not defend the upper part merely, as is the case with claws, but embrace the under parts also, forming a complete case or hoof.

With respect to Linnaeus, however, it must be observed, that although he has followed Ray in placing the elephant in the unguiculate group of quadrupeds, he has not overlooked the great natural divisions which the latter naturalist adopted from Aristotle; and his Ungulata is the more natural in the degree in which it approaches the corresponding group in the Aristotelian system.

I now proceed to the arrangement of the Mammalia proposed by Cuviee in the last edition of his classical work entitled lLe R&gne Animal distribui d'apr&s son organisation.'

Adopting the same threefold primary division of the class Mammalia as his predecessors, Cuvier subdivides it into

1 Aristotle knew that the Cetacea were mammiferous: 'ra' (Si 860 /ifa ixaaroii) '8' ivros, wairep 8e\<pls.'

a From the Systema Naiurre, ed. XII. Holmiae, Tom. I. p. 24.

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