90 Annals of the Carnegie Museum. 



in a corresponding form in the Tropical. The very fact of its survival 

 under the changed conditions of environment indicates a certain 

 plasticity in organization, which may continue in effect, and permit it 

 to vary still further under the slightly different conditions encountered 

 in extending its range. Thus we may explain the case of Atlapetes, 

 for instance, which, with its thirty-six completely and incompletely 

 differentiated forms, was probably derived from a much smaller group 

 of Tropical ancestors. On the other hand, it is conceivable, indeed 

 probable, that in some cases the same Tropical Zone species may have 

 given rise to two or more Subtropical Zone forms in different parts of 

 its range. It seems unlikely, judging by analogy, that certain Sub- 

 tropical forms of wide range have had a simultaneous development 

 in all parts of their range. In such case it would imply that the " orig- 

 inal stock forms" from which they sprang must have had a corres- 

 pondingly wide range, and that the causes which operated to produce 

 the new forms were applied at approximately the same time, and with 

 precisely the same results throughout. 



Once a new Subtropical form had been developed and become estab- 

 lished in a region undergoing elevation, it would naturally tend to 

 extend its range as the land came up to the proper level on either side, 

 and thus the new ground would be- occupied before other competing 

 forms could be developed from the Tropical Zone. Development by 

 parallelism may have occurred, it is true, among the forms of the 

 Subtropical Zone, but it is far more likely that latitudinal extension 

 of range, involving renewed adjustment to varying conditions, com- 

 petition with allied forms, and sometimes virtual or actual isolation 

 eventually, has played a most important part in the evolution of the 

 life of this zone as we know it today, and is largely responsible for 

 the number and diversity of the existing forms which characterize it. 



If this be granted, it follows that the centers of adaptive radiation 

 for the Subtropical fauna must have been located in those parts of the 

 zone which were the first to rise above the altitude of 5,000 feet, or 

 where there were mountains at least as high already in existence when 

 its evolution began. Moreover, the farther the fauna advanced from 

 these centers towards the periphery of its range, the more greatly 

 would it tend to become modified and impoverished. 



The Subtropical Fauna of the Santa Marta Region. — We now have 

 to explain, in view of the foregoing considerations, the peculiarities 



