Compare this with the following sentence, where groups of words are joined.
The work has been much studied, and has been much discussed.
In the following sentence the insertion of a comma would change the meaning.
On this shelf you will put books and pamphlets published in the present year.
As the sentence stands, "published in the present year" applies both to books and to pamphlets: books published in the present year, and pamphlets published in the present year. If there were a comma before "and," the meaning would be: "On this shelf you will put books of any date, and pamphlets of the present year."
(b) When "and" joins the separate words of a series of three or more words, a comma is placed before it.
Trees, and bridges, and houses, were swept down by the flooded stream.
(c) But where the different words are intended to be combined quickly, so as to present to the mind only one picture, they would be spoken without any pause, and in writing must not be separated by any point.
Whirling and boiling and roaring like thunder, the stream came down upon them.
(d) Two of the words of the series may be more closely connected with one another than with the other words of the series, and are, therefore, not to be separated by any point.