GEOGRAPHIC VARIATION IN BIRDS. 



WITH ESPECIAL REFERENCE TO THE EFFECTS OF HUMIDITY. 



Part I — Historical. 



Part II — Dichromatism. 



Part III — Sporadic Melanism. 



Part IV — Experimental. — Hylocichla and Zonotrichia. 



Part V — Experimental. — Scardafella. 



A — Normal Variation in the genus Scardafella. 



-B— The Effect of Humidity on Scardafella inca. 



C — Summary of the Effect of Humidity. 



D — Significance in respect to Direction of Evolution. 



E — Correlation with Natural Selection. 



F — Correlation with Taxonomy. 



G — Correlation with Organic Selection. 



Part I. — Historical. 



It has long been known that many mammals, birds ana rep- 

 tiles inhabiting a moist, humid region show a much darker or 

 increased pigmentation of the hair, feathers or scales than indi- 

 viduals from dryer localities. In cases where there are barriers 

 isolating the humid forms, they are recognized as of specific or 

 of subspecific value. Even where perfectly continuous intergra- 

 dations are found, reflecting the gradual transition from lesser 

 to greater humidity, each extreme type of the series, and occa- 

 sionally several central forms, are differentiated by the system- 

 atist. 



Correlated with this darkening of the epidermal structures, 

 is frequently a distinction in point of size, either of the body and 

 skeleton as a whole, or superficially, as of longer or shorter 

 feathers of the wings and tail. The two best known and most 

 often quoted examples among birds in our own country are Co- 

 linus and Melospiza — the bob-whites and song sparrows. Cer- 

 tain species of the mammalian genera Cants and Sciurus — wolves 

 and squirrels, show this regional or climatal variation well, and 

 among the reptilia, Eutaenia and Crotalus — the garter and rattle 

 snakes — are especially susceptible to climatic influences. 



These facts of distributional variation were enunciated anew 

 in a recent interesting discussion in Science, from which I shall 

 quote a few significant paragraphs. 



