Henry Sidgwick (1838-1900) on Duties to Animals

[I]t is not quite clear whether we owe benevolence to men alone, or to other animals also. That is, there is a general agreement that we ought to treat all animals with kindness, so far as to avoid causing them unnecessary pain; but it is questioned whether this is directly due to sentient beings as such, or merely prescribed as a means of cultivating kindly dispositions towards men. Intuitional moralists of repute have maintained this latter view: I think, however, that Common Sense is disposed to regard this as a hard-hearted paradox, and to hold with Bentham that the pain of animals is per se to be avoided.

(Henry Sidgwick, The Methods of Ethics, 7th ed. [Indianapolis: Hackett Publishing Company, 1981], bk. III, chap. IV, sec. 2, p. 241 [first published in 1907; 1st ed. published in 1874])