1914J 
Jackson: Land Vertebrates of Ridgeway Bog 
39 
the ‘‘balance of nature” (cf. Adams, 1913, p. 28) has been over- 
turned through the agency of man, and the origin of the process 
of distribution is, in a measure, understood; that similar processes 
may have their origin through different agencies seems entirely 
within the range of probability. Basing a decision upon the 
location of greatest abundance of individuals, one might correctly 
locate the North American centers of dispersal of the English 
sparrow and the starling, even though the history of these forms 
in North America were unknown; on the other hand, should one, 
unmindful of the past history of the species, attempt to locate the 
origin of distribution of the English sparrow solely by the location 
of greatest abundance of individuals, one would undoubtedly fall 
into error and would probably locate it in northeastern United 
States. Difficulty is always encountered in determining the 
source of ingression of any plant or animal into a region unless its 
gradual extension of range has been actually observed. The 
ingression of some species into the region of Ridgeway Bog has 
been noted in recent years, but for the most part in determining 
the source of ingression I have relied chiefly upon convergence 
of lines of dispersal, limits of geographic range, biogeographical 
affinities, and location of closely related forms. In my endeavor 
to locate the centers of dispersal as accurately as possible I have 
compiled maps of the geographic range of each species of verte- 
brate found in Ridgeway Bog; in the case of the plants I have 
depended largely upon the distribution maps published by Hough 
(1907) and Sudworth (1913) and upon descriptive geographic- 
ranges from diverse, but reliable, sources. 
The biota of any region may have ingressed from one center of 
dispersal or from many, or it may be partly endemic. Often it 
appears to be simple in character and to show one general source of 
ingression which is easily analyzed; again it is mixed and complex, 
and shows many sources of ingression as if the plants and animals 
entered the region in a hit-or-miss fashion. A careful analysis 
of any such region will usually induce to the conclusion that in- 
gression is regular and orderly; that during a certain period of 
the history of the region, ingression is in general from one source, 
during another period, from another source; that as physical, 
physiographical, or ecological conditions change, a biota best 
adapted, physiologically and morphologically, to meet this change 
