This caught my eye because it's a nice combination of three things I like: murals, ads faded on the side of brick buildings, and graffiti. Unfortunately, the graffiti isn't really inspiring, but what can I do?
The quote on the mural reads "Indication of harm, not proof of harm, is our call to action" and is a quote from Rachel Carson. To the left of the center you can see a sign saying "Rachel Carson was right". I had no idea who Rachel Carson was when I saw this, so I had to come home and do my research...which I will now share with you fine folks.
Rachel Carson was born in 1907 in Pennsylvania and studied marine biology (including a stint here in MA at Woods Hole). She wrote articles on conservation and conducted research for the government, and in her free time she took her government research and "turned it into prose", producing first articles and then books. I found this quote on the website, and I think it sums up her writings nicely: "Embedded within all of Carson's writing was the view that human beings were but one part of nature distinguished primarily by their power to alter it, in some cases irreversibly."
During WWII she started to warn about the misuse of pesticides in her writing, and called for a change in how humankind viewed the natural world. She was challenged by many - including the government and the chemical industry - but continued to speak up about issues of human health and the environment until she died after a long battle with breast cancer.
It's not surprising that you'll find a mural dedicated to an ecologist in Cambridge - Cambridge is a very eco-conscious city and frequently seems to be a step ahead of Boston in that department.