Commentary on Meditations: B4:21-22

You may ask how, if souls live on, the air can accommodate them all from the beginning of time. Well, how does the earth accommodate all those bodies buried in it over the same eternity? Just as here on earth, once bodies have kept their residence for whatever time, their change and decomposition makes room for other bodies, so it is with souls migrated to the air. They continue for a time, then change, dissolve, and take fire as they are assumed into the generative principle of the Whole: in this way they make room for successive residents. Such would be one's answer on the assumption that souls do live on.

We should consider, though, not only the multitude of bodies thus buried, but also the number of animals eaten every day by us and other creatures - a huge quantity consumed and in a sense buried in the bodies of those who feed on them. And yet there is room for them, because they are reduced to blood and changed into the elements of air and fire. How to investigate the truth of this? By distinguishing the material and the causal.

No wandering. In every impulse, give what is right: in every thought, stick to what is certain.

In this passage, Marcus seems to reflect on the aspect of existence and how Nature loves to change. He focuses his mind on the problem of an earth and atmosphere accommodating all the elements. While it may seem that the earth and air might become full of souls and dead bodies, he rightly comprehends that elements decompose to the point "the generative principle of the Whole" or in a word, pneuma.

In the second chapter, it is almost as if he snaps back to it, telling himself to not think about such matters. He tells himself to not wander anymore.  Rather, he should focus his time and effort on what action ought to be done and to focus on what is up to him.

(see also Citadel, p. 29, 41, 45, 186)