“Was there no possibility of saving mankind from self-destruction? The extinct animal species had been forced unwittingly towards elimination, but in man, life for the first time, had mustered sufficient consciousness to recognise dangerous trends in his development. What were these dangers? Progress of science, man's increasing technical mastery? No, the real danger threatening man lay in those gigantic social and political organisations, being an end in themselves instead of means, using the instruments of education and propaganda to suppress individual judgement by creating instinctive mass reaction. These organisations hinder man's spiritual development, reduce the capacity for moral judgement to a low level, and hand over society to the intense lust for power of a few individuals.”

Variant: We commonly say in the trade that the most dangerous animal in a zoo is Man.
Source: Life of Pi

Source: A Raisin in the Sun: The Unfilmed Original Screenplay

Bk. X, ch. 17
War and Peace (1865–1867; 1869)

Source: On Aggression (1963), Ch. XII : On the Virtue of Scientific Humility
Context: We are the highest achievement reached so far by the great constructors of evolution. We are their "latest" but certainly not their last word. The scientist must not regard anything as absolute, not even the laws of pure reason. He must remain aware of the great fact, discovered by Heraclitus, that nothing whatever really remains the same even for one moment, but that everything is perpetually changing. To regard man, the most ephemeral and rapidly evolving of all species, as the final and unsurpassable achievement of creation, especially at his present-day particularly dangerous and disagreeable stage of development, is certainly the most arrogant and dangerous of all untenable doctrines. If I thought of man as the final image of God, I should not know what to think of God. But when I consider that our ancestors, at a time fairly recent in relation to the earth's history, were perfectly ordinary apes, closely related to chimpanzees, I see a glimmer of hope. It does not require very great optimism to assume that from us human beings something better and higher may evolve. Far from seeing in man the irrevocable and unsurpassable image of God, I assert – more modestly and, I believe, in greater awe of the Creation and its infinite possibilities – that the long-sought missing link between animals and the really humane being is ourselves!

University of Havana address (2005)
Context: I would dare say that today this species is facing a very real and true danger of extinction, and no one can be sure, listen to this well, no one can be sure that it will survive this danger.
Well, the fact that the species would not survive was discussed about 2,000 years ago. I remember that when I was a student I heard of the Apocalypse, a book of prophesy in the Bible. Apparently, 2000 years ago someone realized that this weak species could one day disappear.

As quoted in Nikos Kazantzakis (1968) by Helen Kazantzakis, p. 529
Context: Having seen that I was not capable of using all my resources in political action, I returned to my literary activity. There lay the the battlefield suited to my temperament. I wanted to make my novels the extension of my own father's struggle for liberty. But gradually, as I kept deepening my responsibility as a writer, the human problem came to overshadow political and social questions. All the political, social, and economic improvements, all the technical progress cannot have any regenerating significance, so long as our inner life remains as it is at present. The more the intelligence unveils and violates the secrets of Nature, the more the danger increases and the heart shrinks.

Part Third: The Lighthouse, Ch. 8
Nostromo (1904)

Source: Capitalism and Modern Social Theory (1971), pp. 15-16.

The Concept of the Political (1927)