TYPI.S OF NATURAL GROUPS. 55 



series he has formed, and which cannot be made to fall into place 

 by any contrivance he can devise." 



35. The difficulties which are thus detailed as existing in the 

 classification of Insects, are equally felt in every other depart- 

 ment of Zo<'h>L r y; and they result from this, that every natural 

 group or assemblage of species, united by certain characters com- 

 mon to all, is connected, not with two groups merely, one above 

 and the other below it, but with several; and that, of the different 

 modifications which these characters present, a large part are such 

 as to form the transitions from one to another. In every natural 

 assemblage, there is some one which presents the characters that 

 are common to them, in a more remarkable and complete manner 

 than the rest and this is called the type of the group. Thus, 

 each genus has its typical species; each family its typical genus; 

 each order its typical family ; and each class its typical order : 

 the type, in each instance, being that subdivision to which our 

 minds naturally revert, as best exhibiting the characters that be- 

 long to the entire group. We may regard the type of each genus 

 as forming its centre; and the other species as having their places 

 at a greater or less distance from it, according as they differ from 

 it more or less in their respective characters. Some there are 

 which do not depart widely from the type ; whilst there are 

 others which differ from it to such a degree, that we might have 

 failed to recognise the connection, if it were not completely shown 

 by intermediate links. These are called aberrant forms. Now, 

 we will suppose the centres or types of these groups to be 

 spread out over a surface, so that each should be surrounded by 

 a number of others most nearly allied to it ; we should then find, 

 that we might arrange the different species round these centres 

 respectively, so as to form groups, of which every one shall come 

 into contact with others, by species that blend, more or less com- 

 pletely, the characters of both. 



36. The following illustration will, it is hoped, make this 

 tnatter plain. We will suppose a large territory occupied by a 

 number of distinct tribes of people, whose respective possessions 

 are not separated by any very distinct bounds, but of which every 



