RETROGRESSION. 179 



We may illustrate this neat mathematical arrangement 

 of animals by the Vertebrates : 



Subtypical Birdg Mammals Typical 



Group I A I Group 



Aberrant Group 



(3) The third, and perhaps the most fundamental, 

 proposition of the circular system is, the animals con- 

 tained in any given circular group are 'symbolically or 

 analogically represented ' by the animals contained in 

 each and every other circular group in the animal king- 

 dom. In order to understand this proposition fully, a 

 few words must first be said on what Swainson under-, 

 stood by his 'typical,' 'sub-typical,' and 'aberrant' 

 groups. In any given series of animals the 'typical' 

 group is that comprising those forms which possess the 

 largest number of the distinctive characters peculiar or 

 common to the whole series. The ' sub-typical ' group 

 comprises those forms which exhibit a smaller proportion 

 of the presumed distinctive characters of the series. 

 Finally, the 'aberrant' group comprises forms which 



