10 INTRODUCTION 



Hence a system of classification will be natural or artificial, 

 according as it does or does not keep steadily in view this 

 principle, with which all sound results must be consistent. 



When it has been decided that a number of genera possess 

 so much mutual resemblance in structure that they may be 

 regarded with probability as constituting a distinct branch of 

 the genealogical tree of the Lepidoptera (and this is what is 

 meant by family grouping), the question must arise, which of 

 these genera are older than the others, and which are the 

 latest developments. It may often be difficult and sometimes 

 impossible to answer this, but in most cases an approximate 

 result can be reached by a consideration of the following laws, 

 viz. : 



(1) No new organ can be produced except as a modification 

 of some previously existing structure. 



(2) A lost organ cannot be regained. 



(3) A rudimentary organ is rarely redeveloped. 



To the first of these laws certain monstrosities offer an 

 apparent (though not real) exception, but as there is no 

 evidence to show that such monstrosities are ever reproduced 

 under natural circumstances, they may be omitted from 

 calculation. 



Certain other considerations may likewise be of assistance. 

 A large genus, especially if also of very wide distribution, 

 cannot be a very recent one, since it must have required a 

 long period for the differentiation of numerous species, though 

 it must be remembered that as the genus grows larger, the 

 process may become more rapid, from the increased basis of 

 production. On the other hand, a small genus may be of any 

 age ; but one which is closely related to a large genus will 

 almost always be later than it, and a small genus which is 

 widely distributed must generally be an old one. 



In applying the above-mentioned laws in practice, it must 

 be constantly borne in mind that, because two genera are now 

 more closely allied together than to any other, it does not 

 follow that either is descended from the other; it is very 

 frequently the case that both are equally derived from a third 

 genus now no longer existent ; in such a case they are said to 

 be correlated. Further, when one genus is said to be derived 

 from another, and the earlier genus is rich in species, it is 

 not usually meant that the later genus springs from the more 

 advanced forms of the earlier one, but much more commonly 

 from a species standing very near the bottom of the list. 



