Todd-Caeriker : Birds of Santa Marta Region, Colombia. 103 



characterized more by what it lacks. There is thus a marked falling 

 off in species, even within the limits of this zone, with increase in 

 elevation. Nor is the fauna of the Tropical Zone homogeneous in a 

 latitudinal sense. It appears to be composed in the aggregate of an 

 interesting combination of species of arid and humid proclivities, and 

 only towards the eastern extremity of the region is the fauna pure. 

 The Arid Tropical element in the fauna seems to have entered the 

 region from the east, but there are indications that some of the forms 

 ranged under this head may have entered from the west or south, 

 since they are replaced by distinct forms to the eastward. In such 

 case the forms of the latter class must have been later immigrants 

 than the others, since the lower Magdalena Valley region, from which 

 they entered, is much more recent, geologically speaking, than the 

 Goajira Peninsula. Some of these Arid Tropical forms have ap- 

 parently been unable to pass the belt of humid forest which reaches 

 from sea-level to a high altitude on the northern flank of the Sierra 

 Nevada, while others of this class have succeeded in crossing this 

 'biotic barrier and establishing themselves on the farther side. 



The species of Humid Tropical affinities nearly all seem to have 

 come from the south, being forms which belong to the humid section 

 of the Cauca-Magdalena Fauna, and in the main to that element of 

 this Fauna which is believed to have originally entered it from the 

 east. In order to reach the Santa Marta region* these species must 

 have crossed on a northward extension of the forest belt to occupy 

 the area which they now inhabit, and vyhich is known to be of very 

 recent formation. Hence we infer that their advent has been later 

 than for any of the forms of the Arid Tropical. 



The Santa Marta region is deficient in so many of the characteristic 

 Tropical Zone forms of northern South America that we are justified 

 in concluding that it must at one time, when the evolution of birds 

 was at its height, have been a peninsula, practically cut off by water 

 except on the east, whence it received the greater part of its bird- 

 life originally. Probably this first immigration consisted entirely of 

 ' forms of Arid Tropical origin and affinities. Later on, as land formed 

 to the east and south, other fortiis came in from these directions, in- 

 cluding some with Humid Tropical predilections, and this process is 

 going on today. Under the peculiar conditions which obtain in this 

 comparatively limited area a considerable number of indigenous forms 



