THE PLANT FORMATION 219 



different amounts of water and light. Within each habitat, 

 however, these factors vary more or less, and their variation is 

 reflected in the formation. In consequence, the formation shows 

 corresponding differences in composition, and may be analyzed 

 into a number of different parts. The latter are often charac- 

 terized by different species, which give them a more or less dis- 

 tinct stamp. Close analysis shows, however, that these are all 

 of the same general nature, and that the arrangement is a response 

 to the minor variations of physical factors or to competition. 



233. The historical factor. In many instances the variations in 

 the arrangement of species and individuals are due to historical 

 reasons instead of physical ones. An invading species may enter 

 the formation at one point and spread slowly from this. It rarely 

 or never happens that it spreads quickly and uniformly in all 

 directions. In the development of vegetation, one or more species 

 or individuals may persist as relicts for a long time after their 

 fellows have disappeared. This may be due to the accidents of 

 migration and competition, or to the fact that the plant itself 

 has a certain ancestral or historical quality that enables it to 

 persist. This historical element also explains why distant portions 

 of a large formation may show differences that are independent 

 of habitat. It also throws much light upon the puzzling changes of 

 various isolated areas of it, since these are especially exj)osed to 

 invasion. In regions remote from each other, similar habitats 

 are occupied by similar formations, but the species concerned are 

 largely or entirely different, owing to historical facts of migration 

 or development. 



234. Development and structure. Formations show certain 

 changes and certain differences in composition. These may 

 properly be called activities, or functions, and structures. The 

 formation itself may be regarded as a complex organism which 

 shows both development and structure. Practically all forma- 

 tions are undergoing constant change. These changes are rapid 

 in the period of development, but are slow and almost imper- 

 ceptible after the formation becomes stable. They are charac- 

 teristic of young formations, but occur in small degree in old 



ones. 



The changes which make up development are brought about by 

 the movement and establishment of plants, by their reaction upon 

 the habitat, and by the competition between them. Accordingly, 



