304 ECOLOGY 
a) Governing the limit of local and geographic range.—The geographic 
or local range of any species is limited by the fluctuation of a single 
factor (or factors) beyond the limit tolerated by that species. In non- 
migratory species the limitations are with reference to the activity which 
takes place within the narrowest limits (usually breeding). In migratory 
species this activity limits the range during only a part of the life history. 
b) Governing the distribution area and habitat area (55).—The dis- 
tribution area of a species is the distribution of the complete environ- 
mental complex in which it can live, as determined (1) by the activity 
which takes place within the narrowest limits and the animal’s power 
of migration, and (2) by barriers in which some factor of the complex 
fluctuates beyond the limits of toleration of the species in all periods of 
its life history. 
If these statements are borne out by further investigation it follows 
that every study of animal behavior which is related to measured physical 
factors or to natural environments is directly related to problems of dis- 
tribution. 
III. AGREEMENT BETWEEN PLANTS AND ANIMALS 
In recent years the ecology of plants has received much attention 
and the subject has made great progress. In animal ecology but little. 
progress has been made, and students (and teachers) have been inclined 
to expect relations and conditions in animals parallel with those in plants. 
Little progress has been made, largely because workers have not recog- 
nized the important phenomena in animals as compared with plants. 
I. ECOLOGICAL AGREEMENT OF INDIVIDUALS 
Organisms may be divided on the basis of their ability to move 
about, into sessile or fixed, and motile forms. All organisms are of course 
capable of movement of some sort, even though it be only mechanical 
movement dependent upon turgor. There are also all degrees of ability 
to move from place to place. Some motile plants and animals move 
about only very slowly, and the division of organisms into sessile and 
motile is a somewhat artificial classification, as many forms are difficult 
to place in either group. Some are sessile at one period of their lives 
and motile at another. Comparable difficulty arises, however, in the 
separation of plants from animals. 
The animals with which we, as inland people, are most familiar, 
are the highly motile forms, and the plants with which we are most 
familiar are sessile forms. We are all also somewhat familiar with 
