THE ECOLOGY OF THE UPPER GANGETIC PLAIN. 319
could develop naturally. Such a flora now represents the climatic climax as degraded and modified by man. It is the resultant of the struggle between the vegetation on the one hand developing toward some type of climatic climax forest, and the retrogressive influence of the intense human factors on the other hand, continually interfering with and destroying it. This balance between man and the natural vegetation is very delicate, and may perhaps help to explain many of the very serious human problems of the area. After a series of favourable years, the slightly increased vegetation allows a corres- ponding increase in the human and animal population, and results in increased demands on the plant resources. Then subsequent bad years leave the men and animals with insufficient food, and if many do not die as the direct result of famine, they are left so weakened as to become easy prey to- pestilence, and are again reduced in numbers. In other words, under present conditions, the human and animal population is about as large as the area can support. Improved methods of agriculture, and intelligent protection and conservation of the plant resources appear to the only solution of the problem.
Pioneer monsoon deciduous forest stage. If left to itself, it is very probable that the thorn scrub would in turn be replaced by some form of higher and more mesophytic forest. Schimper concludes (10) that if the annual rainfall is below 90 cm., " xerophi- lous scrub", especially " thorn forest" and "thorn bush," prevails ; with 90 — 150 cm. there is a struggle between "xerophilous woodland" and grassland, with the former prevailing when there are greater heat and longer rainless periods during the vegetative season. With 180 cm. or more rainfall, a high forest is produced. Brandis (1) says that " really thriving forests are only found where the fall exceeds 40 inches, and a rich luxurient vegetation is limited to those belts which have a much higher rainfall. "
The highest type of vegetation about Allahabad, the scrubby xerophytic shrubs and trees, would, I think, correspond roughly to Schimper's " thorn scrub. " Doubtless in most regions treated by Schimper, the vegetation described as climax actually is climatic climax. He was recording situations as they actually are ; some of them at least are not necessarily climatic climaxes but modi- fied claimaxes due to retrogressive influences. To class the thorn scrub of the Upper Gangetic Plain as the true climatic cli- max is a mistake. On Schimper's classification, the rainfall in the area about Allahabad should produce a forest somewhere between ' xerophilous scrub" and "xerophilous woodland ". It is difficult to determine exactly what these terms mean. Probably each writer must provisionally fix his own limits to them. Certainly
