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 



