Evolution and Taxo7iomy 47 



synonyms of these terms. It should be remembered that low- 

 ness or simplicity of structure may be the result of degrada- 

 tion, and hence does not necessarily indicate a primitive or 

 generalized condition. The lice are the lowest of the Hemi- 

 ptera ; but they are by no means the most generalized of the 

 living members of that order. 



Professor Hyatt has pointed out * that specialization may 

 take place in two different ways ; first, by an addition or com- 

 plication of parts, specialization by addition ; second, by a re- 

 duction in the number or in the complexity of parts, special- 

 ization by reduction. These expressions are very convenient 

 in indicating the direction of specialization of an organ or set 

 of organs. 



Another important principle, first pointed out, I believe by 

 Meyrickt is that ' ' When an organ has wholly disappeared in 

 a genus, other genera which originate as offshoots from this 

 genus cannot regain the organ, although they might develop 

 a substitute for it." 



The truth of Meyrick' s law, as this last principle may be 

 termed is obvious when we consider that if a part be wholly 

 lost there is nothing for natural selection to act upon in order 

 to reproduce it. And even if a necessity for the organ should 

 again arise and a substitute be developed for it, it is not at all 

 probable that the substitute would resemble the organ so 

 closely as to be mistaken for it. 



In the application of Meyrick's law care must be taken that 

 comparison be made only between allied forms, i. e., within 

 what may be termed a single line of descent. I recognize the 

 fact that these expressions are indefinite, but I believe no 

 systematist will have doubt as to my meaning. 



I^et me state the matter in another way. The loss of an 

 organ is a character that merely indicates a degree of diver- 

 gence from a primitive type. And so far as any single organ 

 is concerned this stage may be reached in one line of descent 

 very much earlier than in another. In fact the loss of an 

 organ may be correlated in one line of descent with a very 



* Insecia, page 51. 



t Trans. Lond. Ent. Soc. 1S84, page 27 



