“I am a vegetarian because I believe that present-day ethics is founded on that puerile, pre-Darwinian delusion that all other kinds of creatures and all worlds were created explicitly for the hominine species. Vegetarianism is the ethical corollary of evolution. It is simply the expansion of ethics to suit the biological revelations of Charles Darwin. Evolution has taught us the kinship of all creatures.”

"Kim Sharma: New vegan on the block" https://web.archive.org/web/20140113154535/http://articles.timesofindia.indiatimes.com/2009-08-12/news-interviews/28205180_1_vegetarian-diet-kim-sharma-bollywood-stars, The Times of India (12 August 2009).

“Wisconsin: On a Monument to the Pigeon”, p. 109.
A Sand County Almanac, 1949, "Wisconsin: Marshland Elegy," "Wisconsin: The Sand Counties" "Wisconsin: On a Monument to the Pigeon," and "Wisconsin: Flambeau"

The fact that an animal is a human, that is, that he belongs to the hominine species of beings, entitles him, regardless of his imperfections, to some sort of consideration.
Source: Better-World Philosophy: A Sociological Synthesis (1899), The Social Ideal, p. 143

"Ethical Implications of Evolution", p. 323
The Universal Kinship (1906), The Ethical Kinship

Introduction to the Bhagavad-Gita (1944)
Context: I have tried to show that the Perennial Philosophy and its ethical corollaries constitute a Highest Common Factor, present in all the major religions of the world. To affirm this truth has never been more imperatively necessary than at the present time. There will never be enduring peace unless and until human beings come to accept a philosophy of life more adequate to the cosmic and psychological facts than the insane idolatries of nationalism and the advertising man’s apocalyptic faith in Progress towards a mechanized New Jerusalem. All the elements of this philosophy are present, as we have seen, in the traditional religions. But in existing circumstances there is not the slightest chance that any of the traditional religions will obtain universal acceptance. Europeans and Americans will see no reason for being converted to Hinduism, say, or Buddhism. And the people of Asia can hardly be expected to renounce their own traditions for the Christianity professed, often sincerely, by the imperialists who, for four hundred years and more, have been systematically attacking, exploiting, and oppressing, and are now trying to finish off the work of destruction by “educating” them. But happily there is the Highest Common Factor of all religions, the Perennial Philosophy which has always and everywhere been the metaphysical system of prophets, saints and sages. It is perfectly possible for people to remain good Christians, Hindus, Buddhists, or Moslems and yet to be united in full agreement on the basic doctrines of the Perennial Philosophy.

As quoted inThe A-V Magazine Vol. 89, No. 1 (January 1981), p. 18, and The Extended Circle : A Dictionary of Humane Thought (1985) by Jon Wynne-Tyson, p. 75; this has been cited to "Harper's Magazine (1890)" but no occurence prior to the 1981 appearance has been located.
Disputed

Introduction, pp.20-21.
The Causes of Evolution (1932)

Source: Why Darwin Matters: The Case Against Intelligent Design

Science and Spirit interview (2004)
Context: All life has a kind of seamlessness. All creatures have to be aware of their environment, and there has been an evolution of the capacities needed for detecting increasingly complex stimuli. I have no problem calling this "meaning," since all creatures pick out meaningful facets of their environment. For the first creatures, these facets were physical and mediated by receptor proteins. Sperm and eggs find each other by protein shapes; photosynthetic bacteria find light by protein shapes. The impetus to figure out what's going on is still very much programmed into our highly complex brains.