Social evolution needs to be examined in relation to the distinct environments of each society. It is the structures of society and their functions, in their origin, development and decline that sociology is concerned with. Societies can also be arranged into classes and subclasses. Society's universal, general and specific structures an functions need to be considered. Overall, Spencer seems to see many useful parallel's between the natural sciences and the science of society.
Society is a collection of individuals.
But in order for the collection of individuals to be a society, the arrangement of them must not be temporary. The permanence among the component parts is what is important. Society is made up of discrete units, but the seeming concreteness of the aggregate is implied because of the maintenance of a similar structure from generation to generation. The maintenance of a similar structure is what yields the idea of society. What interests Spencer is the constancy of the distribution of parts that are the result of settled life.