Todd-Carriker : Birds of Santa Marta Region, Colombia. 89 



facts is unescapable, but before considering the case further we propose 

 to Offer a few general remarks on the 



Origin of the Subtropical Zone Fauna. — The significance of the 

 ■restriction of the Subtropical Zone to the latitudinal limits of the 

 Tropical,- and of its failure to reach sea-level at any point, seems en- 

 tirely to have escaped attention until very recently. The case has no 

 parallel whatever in. the Nearctic Region, where all the various life- 

 zones are primarily latitudinal in character. The fact that the Sub- 

 tropical Zone is wholly altitudinal, and co-extensive with the Trop- 

 ical Zone, indicates that its life, taken as a whole, must have been 

 derived from that of the latter, as Dr. Chapman has shown. But 

 when we come to trace the origin of any given Subtropical form or 

 group we are apt to encounter difficulties.. The profound changes 

 which so many of the Subtropical forms must have undergone in the 

 transition, judging by comparison and analogy, sufficiently attest the 

 potency of the influence exerted by temperature and humidity in modi- 

 fying external characters. Often these modifying influences have re- 

 sulted in the evolution of distinct genera, confined to the Subtropics. 

 So far, indeed, has this evolution proceeded in some cases, as for 

 instance among the hummingbirds, that we are frequently at a loss for 

 a clue to the lines it has followed. If we could be sure that all the 

 Slibtropical forms in a given region had actually been derived from 

 the existing Tropical forms living immediately below them the prob- 

 lem would be simple by comparison, but .such an assumption would 

 probably seldom be justified. There is no question that the physi- 

 ographic and climatic changes which are known to have taken place 

 in tropical America during Tertiary and Quaternary times must have 

 caused much shifting of its bird-population, and consequent modifica- 

 tion and extinction of many species. At the present time we often 

 find what appears to be the nearest ally of a Subtropical Zone form 

 in a more or less remote part of the Tropical Zone. Not until the 

 ranges of all the various forms involved have been accurately worked 

 out and carefully plotted, and their inter-relationships studied in more 

 detail, will we be in a position to even approximate a solution of this 

 question. Much preliminary work thus remains to be done if valid 

 conclusions are ever to be reached. 



Whatever results such a study may yield, we need not assume that 

 every Subtropical form, as such, must have had its immediate origin 



