^ 1912 J Trotter, Relation of Genera to Faunal Areas. 163 



breeding area of these species would seem to indicate, as in the 

 case of the Thrushes above cited, a slow movement of one species 

 upon the heels of another, overtaking one another in their gradual 

 spread, and it is probable that the initial movement was made up 

 of a less number of forms than is represented by the existing species. 



The distribution of color has considerable significance as an 

 indication of descent. The almost universal presence of white 

 blotches on the tail feathers among the species of Dendroica and 

 the presence of wing bars must have been the fundamental color 

 marks of the common ancestral type. And it seems to me, further, 

 that certain primitive varieties of this common ancestor are indi- 

 cated by the greater likeness among some of the existing species. 

 The similarity of such species as striata and castanea in the autum- 

 nal phase of plumage, and the streaky, brownish young of eoronata, 

 tigrina, and palmarum; the head patch common to some forms, 

 the throat patch common to others in the adult plumage, the rump 

 spot and other markings, are very evident features of some com- 

 munity of descent. 



All this, however, is not to the point or purpose of the present 

 paper. What I wish to show here is that a genus like Dendroica 

 possesses evidence in the large number of its specific and varietal 

 forms and in their wide extra-limital distribution of a disruption 

 of some tropical or sub-tropical ancestral type at a remote time 

 compared with such a genus as Hylocichla. Geologically we might 

 express this by saying that the primitive specific types of Den- 

 droica were of late Tertiary origin, whilst the Hylocichline type 

 was broken up into specific forms during the Pleistocene. Birds 

 have their geological history as well as do mammals and other 

 forms of life and as we have not been able so far to find their fossil 

 remains we must look for traces of this history in the specific 

 characters and in the facts of geographical distribution. 



Professor Osborne has cited the case of a mammoth ^ in the 

 stomach of which were found the remains of flowering plants and 

 grasses belonging to species that are still growing in northern 

 meadows. Conservative estimates as to the time when this 

 animal lived would carry us back some twenty-five or thirty 



'■ Osborne — "The Age of Mammals," page 420. 



