1 32 ZOOLOGY. 



species it must be shown that there are certain constant 

 distinctions between them, and (if possible) that these 

 distinctions are kept up by an absence of natural inter- 

 breeding. 



8. Genera, Families and Sub-orders. There are about 

 twenty species of the genus Lepus, in different parts of the 

 world, differing from one another much in the same way as 

 do the rabbit and hare. They all agree in possessing the 

 same dental formula (chap, vii., 18), in having five digits on 

 the fore-limb and four on the hind, in having an imperfect 

 clavicle, and in many minor points. These, then, are the 

 generic characters. / 



The most nearly allied genus to Lepus is Lagomys, of/ 

 which there are about a dozen living species, inhabiting 

 mountainous districts (the picas or tailless hares). This 

 genus is characterized by having only one or two (not 

 three) premolars in the upper jaw, a complete clavicle 

 extending from scapula to sternum, a second caecum to 

 its intestine, and no visible tail. 



These differences are sufficiently great, in the opinion of 

 systematic zoologists, to justify the placing of these genera 

 in separate families Leporidce and Lagomyidce. (Usually 

 a number of genera constitute a family.) 



Both Lepus and Lagomys agree in having two pairs of 

 incisors in the upper jaw : this character marks them off so 

 sharply from the most nearly allied families, that these two 

 constitute by themselves a sub-order Lagomorpha. 



If, instead of starting with the rabbit we had taken the 

 rat or mouse, we might similarly have gone from species to 

 genus, genera to family, families to sub-order at each 

 step taking in animals less and less closely like the one 

 we started with. In this case we should get the sub-order 

 Myomorpha. Similarly the squirrel would serve as a type 

 for the sub-order Sciurimorpha, and the porcupine for the 

 sub-order Hystricomorpha. 



9. Order and Class. The animals of these four sub- 

 orders all agree in a general gnawing habit, in having 

 persistent-pulp incisors, in having no canine teeth, and in 



