250 STUDY OF NATURAL HISTORY. 



having ascertained such a fact as this, through an 

 indefinite number of groups, our next business is to 

 see how far it can be traced in groups of larger di- 

 mensions; and finally, whether it is prevalent in 

 quadrupeds, birds, and other vertebrated animals. 

 Should we succeed in this, we obviously demon- 

 strate that, through all her variations, nature has 

 preserved, at least in one instance, a definite plan of 

 variation, consisting in this, that in every natural 

 group she gives to one of its types a preponderance 

 of tail, or caudal appendages representing a tail. 

 We maintain not, here, that such is actually the fact ; 

 we are merely stating a case, to illustrate the mode 

 of generalising the variation of characters just re- 

 commended, and thereby simplifying the diagnosis 

 by which the different forms in nature are to be 

 distinguished. That this species of generalisation is 

 not impracticable, at least in ornithology, has been 

 clearly demonstrated in numerous groups defined on 

 these principles in the Northern Zoology. And it 

 follows, as a matter of induction, that if the vari- 

 ations of one extensive class of animals are regulated 

 by certain general laws, manifest in all the groups 

 of that class, the same will be discovered in other 

 portions of nature, so soon as they have been investi- 

 gated with sufficient attention to such circumstances. 

 (174.) Yet, although no general rules will here 

 be laid down for the discovery or selection of 

 essential characters, experience has shown that they 

 may be derived with more chance of success from 

 some circumstances than from others. It becomes 

 desirable, therefore, to state what these circum- 

 stances are, and to trace the influence they possess 



