No. 571] TERRESTRIAL ASSOCIATIONS 



421 



petitors for food. Physiological, as well as structural, 

 characters, are accompaniments to modifications of habit. 



3. Psychological Characters. — Apparent preference 

 for certain activities, certain habitats, or certain foods, 

 together with peculiar behavior complexes, seem to be of 

 greater importance in removing animals from competi- 

 tion than structural and physiological characters. 

 Highly regulatory habits permit certain animals to ad- 

 just themselves to changing conditions of competition. 



4. Biographical diameters. — Professor Forbes (1909: 

 295-298) discusses the alternative timing of the active 

 period among close competitors for food. (It so happens 

 that the animals mentioned, having almost identical 

 habits, compete with each other in many ways, besides 

 with respect to food.) In the sand prairie it has been 

 found that different species of certain genera, having 

 otherwise the same habits, differ greatly in life-history. 

 Evidence of this biographical adjustment is more or less 

 complete for two species of Arphia (E: 21), two or three 

 species of Hippiscus (E : 21). two species of the milkweed 

 beetle, Tetraopes (E: 47), and three species of Procta- 

 canthus, robber-flies (E: 55). In these genera the term 

 of activity of one species is abruptly followed by that of 

 another, the successive periods usually covering most of 

 the summer season. 



5. Numerical Characters.— When a certain limited 

 food, place of abode, or other desideratum is used by two 

 or more kinds of animals at one time, a numerical adjust- 

 ment is likely to be found among these competing species. 

 The rate of multiplication of each species must be suffi- 

 cient to keep up its numbers, to allow it to hold place 

 with competing species. (Too high rates of multiplica- 

 tion, on the other hand, are disadvantageous because of 

 other influences.) 



(Ill) Protective, Defensive and Concealing Characters 

 1. Structural Characters.-Ammtds have various de- 

 fensive, protective and concealing structures. Stings, 

 beaks, mandibles, teeth, claws, hairs, spines, resemblance 



