To study disturbed, artificial, and “pathological” conditions, without an adequate knowledge of the normal and original conditions of both the organisms and the environment, is an attempt to interpret the abnormal and artificial in terms of itself, rather than in terms of the normal. If, however, the normal is no longer preserved, then its nearest approach should be studied, but with all the more care and caution. With a proper understanding of the normal, the disturbances made by man will be capable of _interpretation in an orderly sequence strictly comparable to that found in the original and natural conditions_. The cutting down and washing of the lands, the draining and filling of depressions, the flooding of the lands, the destruction (or succession) of plant and animal associations (including crop rotation), are processes brought about or practiced by other organisms or animal agencies. An ecological standpoint gives us a consistent, comprehensive orientation of all these natural and “artificial” activities and processes, and shows the unity in all organic responses to the environment. Man’s influence in the main consists of hastening or retarding “natural processes.” Naturalists have for a long time spoken of the “balance of nature” and of the all-pervading influence of any serious disturbance of it. This balance is, of course, only a relative condition, and not absolutely fixed. It swings from one side, then back, sometimes showing considerable amplitude in its swing, then again its moves are very slight, mere tremblings, as it were. But now and then some local catastrophic event occurs which overturns everything, as when a volcano becomes active, or some dominant association takes possession of the field,--as in the case of man,--and a new order is initiated and a new balance is developed. The mongoose in Jamaica, our English sparrow, and rabbits in Australia are the classic examples of the overturning of the local order of nature by the agency of other organisms. Obviously this balance is not a condition limited to any particular locality or group of organisms. Balance is very generally conceded to be of fundamental importance in the study of any species or group of organisms, if its place in the economy of nature is understood. A vast number of the problems of the economic zoölogist are thus problems, not so much of individual or aggregate ecology, but ones in which the balance of the whole local biotic _association_ is concerned.