A few pages earlier, he quotes this from Henry Beston's The Outermost House, which I am sure I have read quoted somewhere else. I wish I knew where.*
We need another and a wiser and perhaps more mystical concept of animals... We patronise them for their incompleteness, for their tragic fate of having taken form so far below ourselves. And therein we err, and greatly err. For the animals shall not be measured by man. In a world older and more complete than ours they move finished and complete, gifted with extensions of the senses we have lost or never attained, living by voices we shall never hear. They are not brethren, they are not underlings; they are other nations, caught with ourselves in the net of life and time, fellow prisoners of the splendour and travail of the earth.
*Happy day, an answer almost immediately via a review on Google: the passage is also quoted in Tuna, A Love Story, by Richard Ellis.