I have found this a lot with philosophical debates: the generalized statement of how things are. If you do not accept it, if you have some problems with the premises, to use the language of logic, then you`re in trouble.
"In all social animals, including Man, co-operation and the unity of a group has some foundation in instinct. This is most complete in ants and bees, which apparently are never tempted to anti-social actions and never deviate from devotion to the nest or the hive" (p. 12).
What`s going on in this paragraph? The first sentence frames the context: social animals; it further places man (really, what`s meant is human beings...) under the category of "social animals" and it calls for understanding this category in terms of "instincts".
Now, exactly what counts as co-operation and unity is debatable: critical approaches to science (read Donna Harraway for instance) have shown that we tend to see the animal kingdom through the lenses of our social vision. For a long time, the dominant paradigm in seeing sexual practices in the animal kingdom was that of male domination and possession of the passive female. Feminism comes into the picture: scientists start noticing that females are not passive, but in fact make active choices and select male partners.
My point: what we see as unity or co-operation depends on what we take as such. To what extent these are based in instinct - that`s again highly debatable. Instinct is a very good black box: it provides an explanation for a behavior without really explaining it. These things aside, I am not sure ants and bees never ever exhibit anti-social behavior. Nor am I sure their lifestyle is a `devotion` to the hive. Even the use of the word `temptation` here is highly important: anti-social behavior is like a temptation, a deviation from the norm. So really, social cohesion is the norm. Yet, the parallel with the animal kingdom has a specific function in the text: it frames the explanation of social phenomena within the `natural world`, a world which cannot be challenged because, well, it`s natural!
It shouldn`t come as a suprise that a few pages later, Russell furthers the argument of social cohesion and unity are first and foremost visible in the family, the basic social cell (as Lenin was fond to say...). And, like any good nationalist, he continues:
The teleological development of human communities, from families to tribes, from tribes to clans, from clans to nations... starting from the premise of cooperation and unity as an instinct. The argument of the need to be part of the whole, the need to belong to a group is very suspicious to me (not to mention there`s no clear reason as to why nations and not, say, cities... but I`m not going to cover this side of the critique here). There`s a functionalist explanation of it which I find important, but incomplete. I`m still not sure cooperation and unity are a natural necessity - nor am I sure what exactly that means. The more I think about it, the more confused I am about "nature" and "instincts".
"Social cohesion, which started with loyalty to a group reinforced by the fear of enemies, grew by processes partly natural and partly deliberate until it reached the vast conglomerations that we now know as nations" (p. 16)