338 PRINCIPLES OF NATURAL ARRANGEMENT. 



regarded as connected with the rest ; yet they would riot seem to 

 be more easily included in any other groups. Now those members 

 of a natural group which most strikingly present a union of all 

 the characters by which it is distinguished, are spoken of as its 

 types ; and those in which these characters are less obvious are 

 termed aberrant members of the group. It is by these, in fact, 

 that natural groups are connected with one another ; for it will 

 generally be found that in the aberrant members of one group, 

 its characters become (as it were) gradually shaded off, until they 

 almost blend with those of the next. To revert to an illustration 

 formerly (. 18) employed; where the countries occupied by 

 two nations are not separated by any marked natural boundary 

 (as a broad river or high chain of mountains), the peculiar cha- 

 racters of these nations, which may be regarded as most strongly 

 exhibited in their respective chief towns, become gradually 

 blended towards the border where they meet ; so that the tran- 

 sition from one to the other is by no means so abrupt, as if the 

 traveller were conveyed at once from the metropolis of each to 

 that of the other. Every natural group, then, may be regarded 

 as a sphere, surrounded by other spheres each representing 

 another group, which touch it at certain points ; the type of each 

 will occupy its centre ; and the aberrant members will be dis- 

 posed in various positions around it, in proportion as they lose 

 its peculiar characters and approach other groups. For example, 

 the group of Lizards is intermediate between that of Serpents and 

 that of Tortoises. There are some Lizards in which the body 

 and tail are greatly lengthened, whilst the legs are shortened, so 

 that the form of the Snake is approached ; and in the common 

 Slow- worm or Blind- worm of this country, the external form is 

 completely that of a Snake, whilst beneath the skin two pairs of 

 small though perfectly-formed legs may be found on careful 

 examination. This, then, is an aberrant form, situated just on 

 the border of both groups, and scarcely having a certain claim to 

 a place in either. On the other side, the Lizards are connected 

 with the Tortoises by an American species commonly known 

 under the name of the Alligator-Tortoise, or Snapping-Turtle, 

 wnich may be considered as a Tortoise with a long Lizard-like 

 neck, legs, and tail, or as a Lizard with a Turtle-shell on its back. 



