NATURAL HISTORY OF THE EIGHTEENTH CENT. 391 



the infinitely numerous objects which are presented to our observation, 

 but the classification which enables us to do this is in reality arbitrary 

 and artificial. Where nature has insensible gradations, we have found 

 it desirable to draw lines which have (but vainly) been supposed to in- 

 dicate hard and fast natural distinctions recognized by Nature herself. 

 There are indeed cases in which it would at first sight appear that nature 

 had set a class distinct apart from the rest. Take the instance of mam- 

 mals and birds, which appear to constitute classes definitely separated 

 by nature. This, however, is merely an illusion caused, says Lamarck, 

 by our ignorance of animals which exist or have existed. The Ornitho- 

 rynchuSy or Duck-billed Platypus (Fig. 179), an aquatic creature found 

 living in Australia, is justly cited by Lamarck as a surviving member of 



FIG. 180. AMPHIOXUS LANCEOLATUS. 



a group which once filled the existing gap between mammals and birds. 

 This creature is four-footed, and covered with hair, but it has spurs 

 like a fowl, and its mouth is like a duck's bill. Even the division of 

 the animal kingdom made by Lamarck himself into the two great and 

 apparently discontinuous sections of Vertebrates and Invertebrates has 

 since his time been rendered much less sharp by the discovery of the 

 little creature called the Lancelet (Amphioxus lanceolatus\ represented 

 in the annexed cut, Fig. 180. Trie amphioxus is now classed along 

 with fishes, but on its first discovery it was regarded as a mollusc. It 

 is in fact a connecting-link between the two classes, and therefore 

 between the Vertebrata and the Invertebrata. It is about 2 inches 

 long ; it has no skull, no brain, no jaw, no limbs, no distinct heart ; 

 but there extends from one extremity to the other a structure called 



