Commentary on Meditations: B9:9

All things which share some common quality tend to their own kind. Everything earthy inclines to earth. Everything watery flows together, and the same with air, so they need physical obstacles to force a separation. Fire rises upwards because of the elemental fire, but is nevertheless so eager to help the ignition of any fire here below that any material which is a little too dry is easily ignited, for the lack of ingredients which hinder combustion.

So too everything which shares in a common intelligent nature tends equally, or yet more so, to its own kind. Proportionate to its superiority over the rest, it is that much readier to mix and blend with its family.

So right from the beginning among the irrational creatures there could be seen hives, flocks, birds rearing their young, a sort of love: already there were animate souls at work there, and in the higher orders an increasingly strong collective bond which is not found in plants or stones or wood. And among the rational creatures there were civic communities, friendships, households, assemblies: and in war treaties and truces. Among yet higher things there exists a sort of unity even at a distance, as with the stars. Thus the upper reaches of the scale of being can effect fellow-feeling even when the members are far apart.

Look then at what is happening now. Only the intelligent creatures have now forgotten that urge to be unified with each other: only here will you see no confluence. They may run from it, but nevertheless they are overtaken: such is the power of nature. Look carefully and you will see what I mean. You are more likely to find earth not returning to earth than a man cut off from man.

What I get out of this passage from Marcus is this: there is a hierarchical order to elements and communities.  The basic elements easily blend with like material.  Dirt easily blends with other dirt.  Water, air and fire are similar in nature.

Moving up the chain, so to speak, we see elements with intelligence too tend to blend easily.  Then when you observe things with greater intelligence, you see a bond of love.  And still you see, among these groups of intelligence, increasing levels of love and sociability: communities, friendships, etc.

His ultimate point is how at the human level, the urge to unite and become one may occasionally be "forgotten" by individuals, but in the long run they will rejoin the community.  So strong is the need to be social, that it would be easier for dirt to not be unified than for humans.  It is a good reminder that the order of the universe is to be one - to be unified.

(see also Citadel p. 43, 231, 271)