2. The biotic factors.
The term "biotic factors" includes all influences traceable to living organisms, but only animals and man are considered in this paper. Undoubtedly man and his domesticated animals are the most important of the biotic factors. For more than 20 centuries the Gangetic Plain has been populated with an agricultural people. It is difficult and perhaps impossible to form any adequate conception of the intensity of the human factor in times past, but since 1850 the population of the 2811 square miles of Allahabad District has fluctuated from 480 to the high level of 543 to the square mile at the 1891 census (8). At present it is about 530 per square mile. To the human population must be added, according to census of 1909, 331 cattle and buffaloes, 123 sheep and goats, and 8 horses and donkeys, or a total population of domestic grazing animals of about 463 per square mile. Therefore the area has to support a total population of about 1,000 per square mile of animals that gain most or all of their food directly from the vegetation.
Man influences the vegetation in a number of ways, mainly by cultivation, by grazing his animals, and by cutting for food and fuel.
Cultivation. The returns of 1907-08 (8) show that about 58 per cent, of the District was under cultivation. This figure probable fairly indicates the extent of cultivated land from year to year in the area immediately surrounding the city of Allahabad. Cultivation causes retrogression of the vegetation, and the more thorough it is, the greater the effect. Wild plants, both annuals and perennials, are rooted up and killed, and their place is taken by annual ruderals, usually native, but in many cases introduced. At the same time cultivation tends to make the area more xerophytic by removing the permanent plant covering, and substituting a cover of perhaps more mesophytic but short-lived annuals. "When the crop cover is harvested the soil is left practically bare, and dries out speedily.
Grazing animals. All of the uncultivated land in the area is closely grazed throughout the year. During the rainy season, the vegetation is able to keep pace with grazing, even though it is constantly kept eaten off close to the ground. As the cold season progresses and the growth of herbaceous vegetation is checked, the effect of overgrazing become more and more evident. Finally, during the dry season all grasses and associated plants are eaten down to the soil surface (Fig. 11). Annuals die under the combined hardships of the grazing and lack of protection. Only perennial grasses and a few other xerophytic herbs with strong perennating organs are able to survive. Where overgrazing progresses still further, even the peren-
