72 Bird - Lore 
of the Desert Song Sparrow (Fig. 3) with that of the Sooty Song Sparrow (Fig. 4). 
The former inhabits the Colorado desert where the annual rainfall averages 
about six inches; the latter lives on the northwest Pacific coast where the 
annual rainfall averages over ninety inches. 
Again, observe that the Mexican Song Sparrow at the southern extremity 
of the range of the species (No. 1, on map) is the smallest race, measuring some 
six inches in length, and that there is a gradual increase in size northward until 
the maximum is reached at the northern extremity of the range of the species, 
where the Aleutian Song Sparrow (Fig. 1) attains a length of nearly nine inches. 
If we compared only the palest Song Sparrow with the darkest, we might well 
believe, so unlike are they, that each form represents a distinct species; but when 
we include in our comparison representatives of all the twenty-three races of 
Song Sparrows we find complete intergradation in color and in size. Nowhere 
can one draw the line. As the climatic conditions under which the birds live 
change, the birds keep pace. Cause and effect go hand in hand. Here we have 
a species in flower, as it were, a single Song Sparrow stalk with its twenty-three 
blossoms, any one of which might make an independent growth as a species 
if it were separated from the parent stem. Doubtless some day the separation 
will come, when we shall have several species of Song Sparrow, each with its group 
of races, but at present we have only one species, divided into some twenty-three 
sub-species or species in process of formation. 
A variety of reasons may be advanced to account for the pronounced geo- 
graphical variations shown by the Song Sparrow. Its wide range indicates 
physical adaptability and ready adjustment to differences in food and habitat. 
Its variations in size, while they conform to the general law of increase in size 
northward, are exceptionally marked, and are not equaled by those of any other 
North American bird,—a further indication of an inherent plasticity. 
The species is comparatively non-migratory. Several races, notably in Cali- 
fornia, are permanently resident, and a number of contiguous and restricted 
areas may there be found each to have its own form of Song Sparrow. Such 
strictly non-migratory species are continuously subjected to the influences of their 
environment, which are heightened by permanent isolation. But even the most 
migratory forms come early and stay late, and are thus in the breeding area for a 
much greater part of the year than, for example, ay Warblers which come in 
May and leave in August. 
But, suggest as we may the various factors which appear to be active. in pro- 
ducing such geographic variations as the Song Sparrows exhibit, they are not 
potent with all birds, even when other things are equal, and it seems probable that 
some species are in an active state of development and readily respond to the in- 
fluences of their surroundings, while others are fixed and make no such response. 
The latter represent older types of birds, which are, so to speak, near or a part 
of the trunk of the bird’s family tree, while the former class includes the birds 
at the terminal branches of this tree. 
