316 Relations within the Species 



quite unable to move in this direction, he was left isolated and ap- 

 parently bewildered, turning to right and left. Never was there such 

 a frustrated mackerel! 



The groups that have arisen in the various ways described above 

 show all gradations of integration and permanency. In brief, we 

 may note that some groups are brought together mechanically and 

 often the individuals have no direct relation to one another. In other 

 groups, such as insect colonies, a definite organization of the mem- 

 bers exists. Some of the aggregations are of a temporary nature, 

 lasting for only a few hours, days, or weeks. Starlings are commonly 

 observed to gather in noisy flocks in the evening and to roost together 

 in certain large trees, belfries, or other favorite spots; but the birds 

 usually disband at daybreak, flying about independently or in small 

 groups during the daylight hours. Bats follow a reverse daily sched- 

 ule, coming together to roost during the day and dispersing during 

 the night. Some aggregations exist only during the breeding season, 

 whereas in other species flocking or herding takes place for the period 

 of migration only. On the other hand, many herds of large mammals 

 and many insect colonies are essentially permanent— outlasting the 

 lives of individual members and remaining in existence until de- 

 stroyed by some unusual environmental change. For a fuller dis- 

 cussion of this subject and of other aspects of animal aggregations, 

 the reader is referred to the more extended treatment by Allee 

 (1931). 



EFFECTS OF INCREASED NUMBERS 



An increase in the abundance of a species originating in any of the 

 ways discussed in the previous section results in consequences of 

 concern to the species itself and also to other interdependent species 

 of the habitat. The repercussions from the numerical increase of one 

 species on other species will be taken up in subsequent chapters. 

 Here we shall consider the results of population increase within the 

 species, dealing first with relations that are generally harmful and 

 later with beneficial relations. Our present discussion will be cen- 

 tered on contemporary effects, that is, on effects within the life span 

 of the individuals or of the populations considered. Long-term ef- 

 fects on the species may be quite different. Competition, for ex- 

 ample, is usually harmful to the individuals concerned, but its selec- 

 tive action in guiding the course of evolution may result in an eventual 

 benefit to the species as a whole. 



