ON THE ART OF CLASSIFYING. 31 
Thus Linnaus adopts six classes, of which the differences, 
though founded on internal structure, have still reference to the 
function of a part which is by no means common to all animals. 
The number of Linnaus’ Insects and Worms that have no heart 
at all, is in fact at least as great as the number of those in which 
it is present. The so-called Zoophytes, and the Intestinal Worms, 
have no heart: in many animals vessels are already present before 
a heart can be found: insects which undergo metamorphosis have 
only a doubtful rudiment of a heart (the so-called dorsal vessel). 
The physiological importance of the heart is consequently not of 
that kind that it should be considered indispensable in the animal 
economy, as appears to be tacitly assumed in this arrangement. 
Moreover, in Amphibia the heart is provided not with one auricle 
only, but with two, and many of the Worms have not merely a 
ventricle, but also an auricle. Still the first four classes are so 
truly characterised and so firmly founded in nature, that we may 
well wonder that they were not in all times recognised, and not 
earlier formed. That clear and accurate insight which charms 
every one, and that simplicity to which all flatter themselves they 
can attain in their own province, are the true characteristics of 
genius. Linnaus was less happy in his two last classes, as we 
will shew more particularly. Moreover, it is not commendable 
that the division is founded upon a single organ or system of 
organs, as, for instance, those of circulation. In this way, indeed, 
artificial divisions may be obtained, but not a natural method. 
(See above, p. 29.) 
The arrangement of Linn aus, in these last times, has under- 
gone various alterations, when men began, after the example of 
Camper, Pauas, Pout, and others, to investigate the internal struc- 
ture of animals: in which investigation Cuvier}, the first anato- 
mist of our age, by his incomparable achievements was especially 
at Leyden in large folio in 1735: it consists of three tables, each containing one of the 
three kingdoms of nature with some notices appended. He begins with the Mineral 
Kingdom and ends with the Animal Kingdom. 
1 GrorGE LEopotD CHRETIEN FREDERIC DacoBERT CuvIER, b. at Montbéliard 
24 Aug. 1769, d. at Paris 13 May 1832. See my biography of this celebrated contem- 
porary in the Bijdragen tot de Natuurkundige Wetenschappen, vitgegeven door H. C. 
van Haut, W. Vrouicx, en G. J. MULDER. vil. 1832. p. 298—333. Very important 
for the history of Cuvimr’s education are the letters addressed by him to C. H. Prarr, 
the friend of his youth, lately published at Kiel by Professor Brun. 
