STATUS OF THE SPECIES 115 
example, which resent the intrusion of other individuals of the species 
into their nesting area, are comparatively rare; while Robins and Chip- 
ping Sparrows, which, though not colonial, often nest near others of 
their kind, are comparatively abundant. The abundance of a species 
is, therefore, doubtless more dependent upon the number of nests 
which, under favorable conditions of environment, constitute the normal 
number for a given area, than upon the supply of food in that area. 
On this subject, Brewster (“Birds of the Cambridge Region,” pp. 62, 
63) remarks, “In my opinion the desire for exclusive possession so con- 
spicuously shown by the male, and often by him alone, is usually 
the direct result of sexual jealousy. This, as is natural, makes him intol- 
erant, during the breeding season, of the near presence of rival males. 
If his concern were chiefly in respect to the food supply, it would be 
equally manifested at every season and towards all birds who subsist 
on the same food that he and his mate require, which is certainly not 
the case.” 
When in possession of all the essential facts, we may, therefore, 
in many instances, present more or less conclusive reasons for the 
success or failure of a species; but there will remain many others which 
are apparently inexplicable. Species will be obseryed, practically alike 
in structure and habits, some of which in incalculable numbers flood 
the land, while others are represented by but a few individuals scattered 
here and there in small colonies or concentrated in a narrow area. 
All other explanations failing, such cases tempt the belief that 
species as well as individuals have a life, that some full of the vigor 
of youth are advancing, while others past their prime are declining; 
that some in short are dominant, and others decadent. Dominant 
species are possessed of the vitality which admits of adaptiveness and 
power to overcome unfavorable conditions to which decadent species 
would succumb. : 
Thus we may think of a dominant species as an expansive force 
which is preéminently fitted to fill, with the utmost precision of ad- 
justment, its own place in nature, and is also constantly pressing against 
all the governing influences which go to make up its environment, to 
increase its number and extend its range. Up to a certain point it meets 
a degree of success in proportion to its mental and physical endowment; 
beyond that it advances or retreats only after a struggle. 
In February, 1895, the South Atlantic States were visited by a 
blizzard which practically annihilated the Bluebirds wintering or resi- 
dent in them, and the following spring the species was absent from large 
areas, where the preceding year it had been common. The loss of a 
beautiful bird, so intimately associated with man, was deplored as 
irreparable, but within three years the species was as abundant as 
ever, so quickly did the expansive force of the species bring it to the 
limit where contact with environment checks further advance. 
Less evident, but in the end of greater significance, is the Bluebird’s 
struggles with the English Sparrow and the European Starling, both 
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