McLuhan seems to have read I.A. Richards’ Practical Criticism (1929) in 1934 during his first term at Cambridge.  In that same Michaelmas term (or the next Michaelmas term in the fall of 1935) he heard Richards lecture on ‘The Philosophy of Rhetoric’.

Page 248 of Practical Criticism reads:

We come by our ideas in three main fashions : by direct interaction with the things they represent, that is, by [direct] experience ; by suggestion from other people ; and by our own intellectual elaboration. Suggestion and elaboration [the second and third of these] have their evident dangers, but are indispensable means of increasing our range of ideas. It is necessary in practice to acquire ideas a great deal faster than we can possibly gain the corresponding experience, and suggestibility and elaboration, though we must make them responsible for our stock responses, are after all the capacities that divide us from the brutes. Suggestion, working primarily through language, hands down to us both a good and an evil heritage. Nine-tenths, at the least, of the ideas and the annexed emotional responses that are passed on by the cinema, the press, friends and relatives, teachers, the clergy (etc)  — to an average child of this century are (…) crude and vague rather than subtle or appropriate. But the very processes by which they are transmitted explain the result. Those who hand them on received them from their fellows. And there is always a loss in transmission which becomes more serious in proportion as what is transmitted is new, delicate and subtle, or departs in any way from what is expected. Ideas and responses which cost too much labour both at the distributing end and at the reception end both for writer and reader are not practicable, as every journalist knows. The economics of the profession do not permit their transmission ; and in any case it would be absurd to ask a million tired readers to sit down and work [at the interpretation of subtle ideas]. It is hard enough to get thirty tired children to sit up, behave and look bright.
A very simple application of the theory of communication shows, then, that any very widespread diffusion of ideas and responses tends towards standardisation, towards a levelling down.

There are many ideas here which McLuhan would come to treat in his own fashion decades later¹:

– the rear-view mirror — “what is expected” is “responsible for our stock responses” and results in “a loss in transmission which becomes more serious in proportion as what is transmitted is new, delicate and subtle”.

– the medium is the message — “the very processes by which they* are transmitted explain the result.” (* “ideas and the annexed emotional responses…”) 

– all media have further media as their environmental context — “Ideas and responses which cost too much labour both at the distributing end and at the reception end both for writer and reader are not practicable, as every journalist knows. The economics of the profession do not permit their [more exacting] transmission; and in any case it would be absurd to ask a million tired readers to sit down and work [at interpretation].” Ideas in print in newspapers in economics…in a galaxy…

– the content of a medium is another medium — “Nine-tenths, at the least, of the ideas and the annexed emotional responses that are passed on (…) are (…) crude and vague rather than subtle or appropriate. (…) A very simple application of the theory of communication shows, then, that any very widespread diffusion of ideas and responses tends towards standardisation, towards a levelling down.”² 

– “the theory of communication” — must study human interaction “working primarily through language” in a range extending from individual experience with “friends and relatives, teachers, the clergy (etc)” to mass media like “the cinema [and] the press”.

–  the opposition of breadth and depth — The “means of increasing our range of ideas” and of acquir[ing] ideas a great deal faster than we can possibly gain the corresponding experience” entail “stock responses” at “both at the distributing end and at the reception end” of communication. Hence, the “widespread diffusion of ideas and responses tends towards standardisation”.

– speed is fundamental to “the theory of communication” — In the diachronic unfolding of history, it becomes “necessary in practice to acquire ideas a great deal faster than we can possibly gain the corresponding experience”. This is enabled first of all through language and then through contact with wider and wider groups of people (itself enabled through advances in transportation, agriculture, industry, trade, etc), then through writing (and all of its associated consequences), then through printing (and all of its associated consequences), etc etc. This ever increasing speed of diffusion is the most important consideration in an investigation of the modalities of communication and their effects: “the very processes by which [ideas] are transmitted explain the result.”