664 



SCIENCE. 



[N. S. Vol. XXII. No. 569. 



of the mammals of the same region exhibit 

 perfectly parallel variations. Nor are 

 these rules restricted to eastern North 

 America, but prevail throughout the north- 

 ern hemisphere, and also in the southern 

 hemisphere, with, however, the conditions 

 reversed, the increase in size and intensity 

 of colors being from the south northward. 

 High mountains in low latitudes represent, 

 in respect to these phenomena, the higher 

 latitudes nearer sea-level. 



It is equally well known that, in con- 

 tinentally dispersed groups, pallid tints ac- 

 company desert areas and arid conditions 

 of climate, and that increase in depth of 

 color, particularly in gray, brown and olive 

 tints, is an inseparable accompaniment of 

 regions of heavy rainfall and a moist cli- 

 mate, so familiarly illustrated in the north- 

 west coast region of North America. Fur- 

 thermore, there are various other areas in 

 the world where the animal inhabitants are 

 collectively characterized by some special 

 feature of coloration, as excessive lightness 

 of color — hoariness, or increase in area of 

 light or white markings in eastern Siberia, 

 or redness or blackness of coloration in 

 parts of Africa, etc. In other words, re- 

 gional areas of peculiar climatic conditions 

 impress upon their animal inhabitants a 

 certain distinctive phase of coloration, de- 

 veloping in some instances wholly new 

 specific types, in others merely forms that 

 intergrade with others of the immediately 

 adjoining districts. 



To return now to our starting point in 

 eastern North America, the variation in 

 size from the north southward is as gradual 

 and continuous as the transition in climatic 

 conditions; there are no barriers, in the 

 ordinary sense, and no abrupt transitions. 

 In conspecific groups the phase inhabiting 

 Canada or the northern border of the 

 United States, in species of wide breeding 

 range, differs more from the phase inhabit- 



ing southern Florida and the Gulf coast 

 than do many congeneric species known to 

 be completely distinct; and were these 

 phases isolated by a wide geographic in- 

 terval they would have to be recognized in 

 nomenclature as distinct species. It would 

 also be the same if they were found living 

 together, the differences between them are 

 so marked. In reality, however, these very 

 distinct phases are merely the extremes of 

 a single continuous intergrading series or 

 unbroken sequence of individuals of one 

 and the same species. 



In passing westward from the Atlantic 

 seaboard to and across the arid interior of 

 the continent, and thence to the Pacific 

 coast and northward to Alaska, other forms 

 of the same conspecific groups may come 

 into view, so distinct from their eastern 

 representatives and from each other, that 

 many of them were originally described as 

 distinct species, and for many years were 

 so recognized, till the accumulation of 

 material from many intermediate points 

 showed them to be connected by insensible 

 gradations over the intermediate regions, 

 and that their true status was that of in- 

 cipient species, or merely geographic forms, 

 distinct enough when birds of distant and 

 limited areas are compared, but inseparable 

 when those of intermediate regions are 

 taken into account. There are also, in 

 these cases, no barriers beyond the gradual 

 climatic transition from one area to an- 

 other. This, at some points, owing to topo- 

 graphic conditions, may be abrupt, but in 

 general is too diffused to prevent the con- 

 tinuous spread of the species. 



In illustration of these points, we may 

 take the downy and hairy woodpeckers 

 (Dryohates pubescens and D. villosus), 

 both species of continental distribution, 

 and each with its half dozen or more com- 

 monly recognized subspecies, varying enor- 

 mously in color and in size when those of 



