IIJ.] INDEPENDENT SIMILARITIES OF STRUCTURE. 103 



distribution, and, as far as we know, independently of con- 

 ditions other tlian some peculiar nature and tendency (as 

 yet unexplained) common to members of such groups, 

 which nature and tendency seem to induce them to vary in 

 certain definite lines or directions which are difTcl-ent in 



THK GREAT SHIELDED GRASSHOri'EU. 



different groups. Thus v.'ith regard to the grouj) of in- 

 sects, of which the walking leaf is a member, Mr. Wallace 

 observes:^* "The mhole family "^"^ of the Phasmidae, or 

 spectres, to which this insect belongs, is more or less imi- 

 tative, and a great number of the species are called * walk- 

 ing-stick insects,' from their singular resemblance to twigs 

 and branches." 



Again, Mr. Wallace " tells us of no less than four kinds 



'1 See " Naturnl Selection," p. G4. 

 ** Tiie Italics arc not ]\lr. Wallace's. 



'•■' "Malay Archipelago," vol. ii., p. 150; and "Natural Selection," p. 

 101. 



