260 LESSONS FKOM NATUEE. [CHAP. VIII. 



the land Planarise of India, has brought forward evidence 

 that a single Planaria is the equivalent not of a segment of 

 a leech but of a whole leech. Yet a leech is the morpho 

 logical equivalent of a whole centipede, lobster, or other 

 higher Annulose animal, and therefore each higher Annulose 

 animal must be regarded as itself a morphological unit, and 

 not an aggregation of such units. 



Moreover, even lateral, vertical and serial homology do not 

 independent exhaust the kinds of likeness (homologies) which 

 Bimiiarities. j^ye arisen independently of descent : for struc 

 tures are continually being discovered (in animals of different 

 kinds) so strikingly alike that their resemblance would 

 naturally be taken, on the theory of evolution, for a sign of 

 genetic affinity, and yet the circumstances under which they 

 occur preclude any such explanation. The resemblance 

 which exists between the ankle-bones of such widely different 

 animals as frogs, and the small African lemurs, termed 

 Galagos, may be taken as an example of such uninherited 

 likeness. In a genus of the frog order (namely, Pelobates), 

 and in the turtle, a bony expansion covers over that hollow 

 at the side of the head which is called the &quot; temporal fossa.&quot; 

 A similar expansion has lately been found to exist in ascertain 

 African animal of the rat order (namely, Lophiomys), though 

 it exists in no other known beast. The resemblance which 

 exists between Pelobates, the turtle, and Lophiomys must be 

 supposed to have been occasioned independently, and not by 

 inheritance. Again, the African ant-eater, the aard-vark 

 (Oryeteropus), has each tooth, though apparently simple, 

 really composed of a closely-set bundle of very fine, long, 

 cylindrical teeth united together side by side. Such a struc 

 ture exists in no other genus of the same class, but is found 

 in the class of fishes namely, in the skate (Mylidbatis). Yet 

 the aard-vark can have no special relation of genetic affinity 

 with these fishes. The shape of the teeth in kangaroos is 

 similar to that of certain shrew-like, insect-eating African 

 beasts (of the genus Macroscelides), which also agree with 

 kangaroos in having the hind-legs and feet much elongated 



