40 Bulletin Wisconsin Natural History Society [Vol. 12, Nos. 1 & 2 
ingresses; and that generally this biota comes from a region which 
has conditions, in toto or in part, similar to those of the region 
ingressed. The change in conditions may be a single factor such 
as light intensity, humidity, evaporation, soil alkalinity, tempera- 
ture, or what not; or it may be a combination of factors; it is 
effective if it reaches the minimum necessary for the existence 
of the plant or animal. ‘‘The habitat relations of organisms show 
that they do not occur promiscuously mixed, even within a small 
area, but that their relations are orderly and definite. In addi- 
tion to the general successional relation attending changes of 
the environment, attention is called to the different kinds of 
organisms in different biotic regions, which make up this suc- 
cession. This habitat uniqueness of the biota in different regions 
favors the independent formation or association of similar habitat 
types from very diverse kinds of biota’’ (Adams, 1905, p. 70). 
A careful study shows that ingression^^ into Ridgeway Bog 
region has taken place from three general directions — south, 
north, and east, and that the biota of any one association has 
ingressed, for the most part, from the same general center of dis- 
persal. The biota of the Aquatic-Sedge Associations is typified 
in a large measure by plants and animals of southern affinities; 
the combined biota of these two associations consists of such 
species as Potajnogeton natans, Nuphar advena, Typha latifoUa, 
Iris versicolor, sedges, Menyanthes trifoliata, Bufo americanus, Rana 
pipiens, Rana septentrionalis, Thamnophis sirtalis, Chrysemys 
helm, Querquedula discors, Porzana Carolina, Actitis macularia. 
Circus hudsonius, Asio flammeus, Agelaius p. arctolegus, Melospiza 
georgiana, Telmatodytes p. iliacus, Microtus perunsylvanicus. Ondatra 
zihethica, Mustela v. letifera, and Blarina hrevicauda. Porzana 
Carolina, Actitis macularia, and Circus hudsonius are probably of 
northern origin, having Palaearctic relations in Porzana porzana, 
Actitis hypoluca, and Circus cyaneus, and upon the strength of this 
evidence I have, with some hesitation, indicated for them a 
northern center of dispersal; it seems, however, possible that they 
may have been driven south by the glaciers and may have since 
ingressed the region from that direction. Mustela v. letifera 
Reference to tables 1 and 2, in which I have indicated the probable source of ingression 
of the majority of the species in Ridgeway Bog, will clarify the statements in the following para- 
graphs. 
