Quentin Smith

[W]e cannot concentrate only on benefactors to humans. Perhaps Peter Singer, the most influential person in promoting the welfare and rights of animals, will ultimately have contributed more to the development of the universe than benefactors merely of humans. Perhaps Singer’s book Animal Liberation will (over the centuries) have increased the happiness, health, and lives of animals to such an extent that it adds up to a greater amount of goodness than the human development that will be the total consequence of (say) Gandhi’s actions.

Quentin Smith, Ethical and Religious Thought in Analytic Philosophy of Language, New Haven, 1997, p. 217