19UJ 
Jackson: Land Vertebrates of Ridgeway Bog 
37 
V. THE SOURCE. OF INGRESSION OF THE BIOTA 
That we may consider the probable source of ingression of the 
biota of Ridgeway Bog region since glacial times it is necessary 
to locate approximately the last center of dispersal of each species 
found within this area; in the case of some species, in order to 
throw light on lines of dispersal, it is essential to locate, as nearly 
as possible, the origin of distribution. The terms “origin of dis- 
tribution” and “center of dispersal” must not be confused; the 
former refers to that place where the species, group of species, or 
mores was first evolved and from which it originally extended 
its geographic range; the latter refers to that place of limited area 
from which it extends its geographic range after a period of static 
or egressive distribution, or after being transferred to this area 
distant from its former geographic range. We may thus have two 
or more centers of dispersal at a given time, but never more than 
one origin of distribution, unless one would be willing to accept 
the fanciful theory that parallel evolution and convergence take 
place in localities remote from one another. Various criteria 
have been used to determine the center of dispersal; these have 
been summarized by Adams as follows: 
1. Location of the greatest differentiation of type. 
2. Location of dominance or great abundance of individuals. 
3. Location of synthetic or closely related forms. (Allen.) 
4. Location of maximum size of individuals. (Ridgway, Allen.) 
5. Location of greatest productiveness and its stability, in crops. 
(Hyde.) 
6. Continuity and convergence of lines of dispersal. 
7. Location of least dependence upon a restricted habitat. 
8. Continuity and directness of individual variations or modifications 
radiating from the center of origin along highways of dispersal. 
9. Direction indicated by biogeographical affinities. 
10. Directionindicatedby annual migration in birds. (Palmen.) (Adams, 
1902, p. 128.) 
The location of greatest productiveness and the location of 
maximum size of individuals are probably of importance in deter- 
mining the center of dispersal of plants; but in determining the 
center of dispersal of vertebrate animals I believe they count for 
little; ecological conditions at the center of dispersal ma^^ come far 
from fulfilling the optimum for the existence of a species which 
