III.] INDEPENDENT SIMILARITIES OF STRUCTURE. 1Q3 



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

 ditions other than 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 different in 



THE GEEAT SHIELDED GRASSHOPPER. 



different groups. Thus with regard to the group of in- 

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

 observes : 21 " The whole family 22 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 23 tells us of no less than four kinds 



21 See " Natural Selection," p. 64. 



22 The Italics are not Mr. Wallace's. 



23 "Malay Archipelago," vol. ii., p. 150; and "Natural Selection," p. 

 104. 



