Émile Durkheim in his book «The Division of Labour in Society» in chapter five is talking about his theory how organic solidarity appeared and how it increased in the modern human world.
So solidarity for Durkheim is the cohesion in the society that binds all people together making them think and understand that they are part of something one and single.
Generally he says that in the beginning there was a mechanical solidarity. It appeared in the pre-modern society and was based on similarity. For example, you are a citizen in Ancient Russia, you have a beard, your skin is white and you believe in Yarilo and there are a lot of people with a same thoughts as you have. Of course, there is a leader of your group, someone doesn’t have a beard, you have gender division, but, in general, these people are bound together because most of them hold same values and experiences. Durkheim calls these tendency – «collective consciousness». So all these things contribute to mechanical solidarity or similarity that hold all people together.
The next type of solidarity mentioned in this chapter is organic solidarity that appeared in modern society. It certainly differs from pre-modern one because people there are different. There is no need to prove that nowadays most of people have different beliefs, various jobs and values. Thus, there is no similarity (but also not complete difference, because there always will be some fundamental thoughts) and what holds people together is their inter-dependence which in simple words – relying on each other.
Due to that definitions and explanations, Durkheim claims that the there are main differences between two societies. First one is the division of labour, because in the modern society we can see tendency of specialisation that creates inter-dependence. Another difference is related with reactions and repercussions. In pre-modern society, because of similarity, leaving your group will cause bad consequences to you, while in modern one different thoughts won’t affect badly on you until the system works.
To explain how mechanical solidarity moved to organic Durkheim refers to dynamic density and interactivity between people. Nowadays, we call this globalisation, but it in past centuries it was just social evolution.
In my opinion, this theory has the place to be, but «father of sociology» doesn’t say any strong arguments about war, because it ruins the idea of inter-dependence. Moreover, this theory can not deal with Marx’s conflict theory in which classes are struggling against each other.