“The species is in a state of transition, one of many. This one began, generally speaking, when the species tried to step apart from nature in order to develop the unique kind of consciousness that is presently your own. That consciousness is not a finished product, however, but one meant to change, [to] evolve and develop. Certain artificial divisions were made along the way that must now be dispensed with. You must return, wiser creatures, to the nature that spawned you - not only as loving caretakers but as partners with the other species of the earth. You must discover once again the spirituality of your biological heritage.”

The Development Hypothesis (1852)
Context: The supporters of the Development Hypothesis... can show that any existing species—animal or vegetable—when placed under conditions different from its previous ones, immediately begins to undergo certain changes fitting it for the new conditions. They can show that in successive generations these changes continue; until, ultimately, the new conditions become the natural ones. They can show that in cultivated plants, in domesticated animals, and in the several races of men, such alterations have taken place. They can show that the degrees of difference so produced are often, as in dogs, greater than those on which distinctions of species are in other cases founded.

The Origin of Humankind (1994)

The portion of this statement, "Love is the only sane and satisfactory answer to the problem of human existence" has been widely quoted alone, resulting in a less reserved expression, and sometimes the portion following it has been as well: "Any society which excludes, relatively, the development of love, must in the long run perish of its own contradiction with the basic necessities of human nature."
The Art of Loving (1956)
Context: Our society is run by a managerial bureaucracy, by professional politicians; people are motivated by mass suggestion, their aim is producing more and consuming more, as purposes in themselves. All activities are subordinated to economic goals, means have become ends; man is an automaton — well fed, well clad, but without any ultimate concern for that which is his peculiarly human quality and function. If man is to be able to love, he must be put in his supreme place. The economic machine must serve him, rather than he serve it. He must be enabled to share experience, to share work, rather than, at best, share in profits. Society must be organized in such a way that man's social, loving nature is not separated from his social existence, but becomes one with it. If it is true, as I have tried to show, that love is the only sane and satisfactory answer to the problem of human existence, then any society which excludes, relatively, the development of love, must in the long run perish of its own contradiction with the basic necessities of human nature. <!-- p. 111 - 112

A Magazine of People and Possibilities interview (1998)

Praelectiones (Lectures, 1744) quoted in Larson (1967:317)

Incognito: The Secret Lives of The Brain

Fundamenta fructificationis (1742). As quoted in John S. Wilkins (2009), "Species: A History of the Idea," University of California Press. p. 72

Address to his household, Yverdon, Switzerland, on his seventy-second birthday (1818-01-12)

They can show that in successive generations these changes continue; until, ultimately, the new conditions become the natural ones. They can show that in cultivated plants, in domesticated animals, and in the several races of men, such alterations have taken place. They can show that the degrees of difference so produced are often, as in dogs, greater than those on which distinctions of species are in other cases founded.
The Development Hypothesis (1852)