THE GENUS CHOBDEILES SWAINSON — OBEBHOLSEB. 13 



a small, somewhat modified ChordeUes, is a more direct offspring of 

 the latter group. 



The real relationship and therefore the origin of the three species 

 of ChordeUes is a problem of some difficulty. It seems probable, 

 however, that ChordeUes virginiccirMS, to have become, in structure, 

 pattern of coloration, and color so well differentiated specifically from 

 both ChordeUes acutipennis and ChordeUes rupestris; to have estab- 

 lished itself entirely outside of South America ; to have spread over 

 and permanently occupied as a summer home so large an area ; and to 

 have become modified into so many geographic races, notwithstand- 

 ing the very brief period in each year during which the modifying in- 

 fluences could be operative, must be a relatively ancient species. On 

 the other hand, ChordeUes acutipennis, less stable than ChordeUes 

 virginianus in structural characters, less dispersed into territory 

 where it is migratory, modified into fewer subspecific forms, and 

 these less strongly differentiated from the parent stock, and still 

 largely resident in South America, seems to be of more recent origin. 

 But ChordeUes rupestris, which is very different from ChordeUes 

 acutipennis in pattern of coloration and in structure, and which 

 occupies approximately the same area in South America, probably, 

 therefore, developed simultaneously from the same ancestor. Fur- 

 thermore, the close superficial resemblance in color and markings 

 which ChordeUes acutipennis acutipennis from northern South 

 America bears to ChordeUes virginianus minor from Jamaica and 

 Cuba is at least suggestive, if not significant. Thus we naturally 

 arrive at the hypothesis that all three species of ChordeUes were de- 

 rived fi-om a conunon ancestor, probably resembling ChordeUes vir- 

 ginianus in structure and ChordeUes acutipevms in coloration, but 

 now extinct, from which ChordeUes virginianus first became differ- 

 entiated, and afterwards both ChordeUes acutipennis and ChordeUes 

 rupestris, the latter so greatly specialized, as it is now seen to be in color 

 pattern, by some unknown agency, possibly individual variation and 

 subsequent fortuitous, temporary segregation. The only region where 

 the two species, ChordeUes virginianus and ChordeUes acutipennis, 

 occupy a common breeding area is in the southwestern United 

 States and extreme northern Mexico. Here the former seems to have 

 immigrated first, and from the north and east; while afterwards 

 ChordeUes acutipennis, then a perfectly distinct species, extended its 

 range northward from southern Mexico until it overlapped that of 

 ChordeUes virginianus, and it seems to be still pushing northward. 



Sequence of forms. — An ideal linear sequence of species and sub- 

 species, which shall show their proper phylogenetic relationship, is, 

 however, desirable, probably out of the question in a complicated 

 group like the genus ChordeUes. As some such arrangement is, of 



