92 COLOR AND CONCEALMENT 
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 inter- 
gradation 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 it if 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 
geographical 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 equalled 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 California, are permanently -resident in contiguous and restricted 
areas. Such strictly non-migratory species are continuously subjected 
to the influences of their environment, which are heightened by perma- 
nent 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, many Warblers which come in May and leave 
in August. 
But, suggest as we may the various factors which appear to be active 
in producing 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 de- 
velopment and readily respond to the influences 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 to 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. 
Color and Concealment.—It is clear that the life of the individual 
bird, and hence the continued existence of its species, depends pri- 
marily on its success in securing food and in escaping from its foes. The 
colors of predaceous species must, therefore, tend to conceal them from 
their prey (aggressive or deceptive colors), while the colors of those 
that are preyed on are designed to conceal them from their enemies 
(protective colors). 
When, however, a bird’s food may be secured without the exercise 
of caution, when it apparently is not preyed upon, or when it is protected 
