The concept of "environment" is certainly difficult and may even be misunderstood; but we have no handy substitute . It seems enough to distinguish between the organism(有机体) and the surrounding environment and to separate forces acting on an organism into those that are internal and biological and those that are external and environmental. But in actual practice this system break down in many ways, because the organism and the environment are constantly interacting so that the environment is modified by the organism and vice versa(反之亦然). In the case of man, the difficulties with environmental concept are even more complicated because we have to deal with man as animal with man as a bearer of culture. If we look at man as an animal and try to yze the environmental forces that are acting on the organism, we find that we have to deal with things like climate, soil, plants, and such-like factors common to all biological situations; but we also find ,always, very important environmental influences that we can only class as "cultural", which modify the physical and biological factors. But man, as we know him, is always a bearer of culture; and if we study human culture, we find that it, in turn, is modified by the environmental factors of climate and geography. We thus easily get into great difficulties from the necessity of viewing culture, at one moment, as a part of the man and, at another moment, as a part of the environment. According to the author, the concept of "environment" is difficult to explain because _______.