suggesting that once in other to account for a stage of evolution it has to reach the next level. in the passage the author does account for 2 stages but then wouldn't that suggest that there has been 3 stages.the true meaning and nature of one stage is only fully revealed once the next stage has been reached.
paragraph two starts by saying
suggesting that it is one of the stages but not the first so since that stage has been reached there is one stage accounted for. namely the one previous to the brain.One important advance was the development of a brain
Then, the next paragraph says
which in terms will close the stage of evolution concerning the development of the brain, thus accounting for the second stage and lastly, the current stage or the one truncated by Socrates is the third stage.Another distinct stage in this evolution is the advent of man and his faculty for discourse