The properties of the units determine the properties of the whole. If two groups are composed of different sorts of individuals, then the groups will differ to the extent that the individuals in the groups differ. What is important are the essential characteristics, the cardinal traits and not the incidental traits. The role of Sociology is to express the relation between the proprieties of the whole and the properties of the units that make up the whole. It should look at what qualities of individuals promote aggregation, as well as those that negate it.
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.