1907] Beebe: Geographic Variation in Birds. 15 



America. Mr. Charles Sheldon writes me as follows concerning 

 the occurrence of the black and the white forms : "I am certain 

 that at some points the same mother has black and white lambs — 

 that a white mother has a black lamb, that a black mother has a 

 white lamb ; at such points, however, none of the sheep are pure 

 white or pure black. Moisture has nothing to do with their color. 

 Where the black forms are found, the climate is the same and 

 as dry as in the habitat of the white sheep. On the north side 

 of the Alaskan range the sheep are uniformly pure white, yet 

 living among glaciers and at an altitude where it is much damper 

 than where the black sheep are found. Indeed, the rainfall is 

 almost constant^ in the mountains in the summer." 



While it appears impossible, in the present state of our 

 knowledge, to correlate with more certainty the foregoing exam- 

 ples of melanism and dichromatism, yet we should keep them all 

 in mind while endeavoring to interpret the results of future field 

 studies or experimental researches along these lines. Whether 

 mutation plays a part in the development of any of them or 

 whether all are to be explained as arising by the accumulation of 

 continuous variations, one thing is certain, it is upon one or both 

 of the methods of work which I have mentioned, that we must 

 depend for the elucidation of problems of this character. The 

 collecting of thousands of skins will be of no service nor will the 

 study of those now in our museums be of any direct use. We 

 must have careful and minute tabulation of the ecological condi- 

 tions under which the phenomena under discussion appear, cor- 

 related with the effects of similar, as well as intensified and 

 modified, climatal and other factors, upon individuals under con- 

 stant observation. 



Part III. — Sporadic Melanism. 



Many records of casual or sporadic melanism are scattered 

 through ornithological literature, of which I shall cite only a 

 few. In 1876 Mr. Ruthven Deane wrote that "melanism is of 

 exceedingly rare occurrence, and but five species have been re- 

 corded on my list: Turdus migratorius, Colaptes auratus, Me- 

 lanerpes erythrocephalus, Orytx virginianus, and TJria grylle." 

 Later, in 1879, he adds the Carolina rail to this list, and since 

 then other writers have considerably increased the number of 

 birds observed in a melanistic state, such as Mniotilta varia and 

 Stiix flammea. Many of these records have reference to wild 

 birds, others to individuals bred, or at least reared, in captivity. 



