He used that phrase in a letter he wrote to Claire 
Booth Luce—saying something to the effect that 
the media ecology of cultures probably need to be 
managed in a systematic way— and the term 
struck me as very useful, since ecology was used 
to mean the study of environments and how 
environments can be healthy and become toxic and 
so on. I thought that his putting the term media in 
front of ecology suggested in a forceful way that 
people studying media were not just studying 
machines and how they work, but the interaction 
between the structure and form of machines and 
the human sensorium. This gave a special kind of 
meaning to the phrase symbolic environment