Social change is more likely to occur in societies where there is mixture of different kinds of people than in societies where people are similar in many ways．The reason for this is that there are more different ways of looking at things present in the first kind of society．There are more ideas, more disagreements in interest, and more groups and organizations with different beliefs．In addition, there is usually a greater worldly interest and greater tolerance in mixed societies．All these factors tend to promote social change by opening more areas of life to decision．In a society where people are quite similar in many ways, there are fewer occasions for people to see the need or the opportunity for change because everything seems to be the same．And although conditions may not be satisfactory, they are at least customary and undisputed.
Within a society, social change is also likely to occur more frequently and more readily in the material aspects of the culture than in the non-material, for example, in technology rather than in values; in what has been learned later in life rather than what was learned early; in the less basic and less emotional aspects of society than in their opposites; in the elements rather than in the complex ones ;in form．rather than in substance; and in elements that are acceptable to the culture rather than in strange elements.
Furthermore, social change is easier if it is gradual．For example, it comes more readily in human relations on a continuous scale rather than one with sharp dichotomies(一分为二)．This is one reason why change has not come more quickly to Black Americans as compared to other American minorities, because of the sharp difference, in appearance between them and their white counterparts.
The passage mainly discuss ______.