III.] INDEPENDENT SIMILARITIES OF STRUCTURE. 101 



type has been developed iu the United States, and this in 

 about a couple of centuries only, and in a vast multitude of 

 individuals of diverse ancestry. The instances here given, 

 however, must suffice, though more could easily be adduced. 

 It may be well now to turn to groups presenting similar 

 variations, not through, but independently of, geographical 

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

 ditions other than some peculiar nature and tendency (as 



THE SIX-SHAFTED BIRD OF PARADISE. 



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 different groups. Thus with regard to the group of 

 insects of which the walking leaf is a member, Mr. 

 Wallace observes : l "The whole family 2 of the Phasmidee, 



1 See "Natural Selection," p. 64. 



2 The italics are not Mr. Wallace's. 



