III.] INDEPENDENT SIMILARITIES OF STRUCTURE. 93 



the curious similarity between a true mouse (Mus deli- 

 catidus) and a small marsupial, pointed out by Mr. Andrew 

 Murray in his work on the " Geographical Distribution of 

 Mammals ;" but instances enough for the present purpose 

 have been already quoted. 



Upper Figure ANTECHNINUS MINUTISSTMUS (implacental). 

 Lower Figure Mus DELICATULUS (placental). 



Additional reasons for believing that similarity of struc- 

 ture is produced by other causes than merely by " Natural 

 Selection" are furnished by certain facts of zoological 

 geography, and by a similarity in the mode of variation 

 being sometimes extended to several species of a genus, 



