*89'-] Lucas, Bird Skeletons from Guadalupe /shn/d. 2IQ 



acteristic species, while man}- of them are separated by a wide 

 gap from their nearest rehitives of the mainhmd, and we may say 

 that in the Gahipagos we see differentiation in its completion, 

 and in Guadalupe in its inception. 



The value of these skeletons lies in the fact that they give us 

 some hints as to the comparative rapidity with which external 

 and internal changes may take place, and it is much to be re- 

 gretted that we possess no good series of skeletons of species 

 common to the island and the continent. 



As the climatic conditions existing at Guadalupe are not very 

 different from those prevailing on the mainland, color differences 

 between subspecies, or even closely allied species, would be 

 largely the result of any innate tendency to variation, while 

 structural differences would be due either to the same cause, or 

 to change of habit produced by restricting the range of individuals 

 to a limited area. Now while a considerable amount of individ- 

 ual variation will be found to exist in any extensive series of 

 specimens of a given species, such differences, aside from those 

 of mere size, are, as a rule, either reversionary in character or 

 due to physiological adaptation, the existing groups of birds, and 

 especially the Passeres, seeming to have become so fixed in their 

 respective types that new morphological departures are extremely 

 rare. It would, therefore, have been very strange had any such 

 departure been found to exist in the five species represented, and 

 it is very evident that the skeletal peculiarities presented by the 

 skeletons under consideration are the result of change of habit 

 due to insulation. 



In order to express the relative proportions of the limbs and 

 sternum and show the amount of their variation in the birds con- 

 sidered, the length of the vertebral column, exclusive of the 

 caudals, was called one hundred, and the various parts compared 

 with this standard. 



The skeletal differences between Polyborus tharus and P. 

 lutosus^ the first on the list, are extremely slight, so sHo-ht in- 

 deed, that judged by them alone there are no grounds for consid- 

 ering the two birds as belonging to two species. That there are 

 no perceptible distinctions between the skeletons of the two spe- 

 cies, is not, however, surprising, for Polyborus tharushc'xw^ wow- 

 migrator}-, the habits of the two birds must be very much alike 

 and there would be no physiological reason for any chan<j-e 



