Of God and His Creatures

The irrationality of dumb animals is apparent from this, first, that they are dumb (alogoi), or devoid of rational speech; secondly, that they are uncivilised, and uncivilisable, except in so far as they partake of the civilisation of man; thirdly that, apart from man, they are racially unprogressive; fourthly, that they are devoid of all idea of morality and religion; fifthly, that there is no inter-breeding between them and even the lowest types of the one incontestably rational animal, man. The indefeasible coexistence of human shape and animal rationality is as well established as any coexistence in physical science. But, setting aside spiders and swallows, it is not true that monkeys, elephants, horses, dogs, and other educable animals, in their respective species, "all behave alike." St Thomas however may claim to speak only of animals in a state of nature, wholly uninfluenced by man.

Of God and His Creatures: 2.82