GROUPS NOT ALWAYS PERFECT. 213 



We may, in this stage of our enquiry, have some 

 reason to suppose that it is a natural one, because it 

 exemplifies the law in question. 



(149.) It has been found, however, that this is 

 not a sufficient verification of a natural series ; and 

 for this reason : in the infinite variety of animal forms, 

 there are so many mutual resemblances, that if our 

 only object is to arrange them in circles, we may 

 combine them in different ways, all of which will 

 wear a prima facie appearance of being more or 

 less circular ; or if any unusual hiatus or gaps appear, 

 we are immediately ready to smooth over the diffi- 

 culty, by concluding that they do not really exist in 

 nature, but only in the paucity of our materials. 

 And we are the more inclined to yield to this per- 

 suasion, since naturalists universally admit that such 

 intervals really do exist in nature, either from the 

 extermination of some animals by man, or from the 

 changes which our earth has undergone. Against 

 this disposition, therefore, to smooth over discrepan- 

 cies in our supposed circle, by attributing them to 

 causes which may or may not be the true ones, 

 the naturalist must frequently contend. He may 

 rest assured that a natural group can only have 

 one series of variation ; and if, by taking away 

 some of its parts, and substituting others, he 

 can still preserve its circular appearance, he has 

 strong reason to doubt which disposition is the 

 natural one. 



(150.) Now, under these circumstances, the ef- 

 fective mode of proceeding is to form two or more 

 of those combinations called genera, and then to com- 

 pare their contents respectively with each and all, 

 p 3 



