St Thomas has seven chapters (XXXIX-XLV) discussing the variety of creatures, why the universe is not uniform but diversified, and how it has come to consist of such diverse components. As regards living creatures, the discussion is familiar to us from Darwin's Origin of Species and the theory of Evolution. St Thomas ventures on a larger question, the origin of all species, inanimate as well as animate. He states and rejects various archaic theories; but the point of supreme interest to the modern mind is never raised. In all the seven chapters there is not one word pointing to evolution. I have been driven to make large omissions, omissions which I feel sure the Saint would have sanctioned, had he been face to face with the cosmogonies of our day. Life is short art is long: the ground of philosophy must not be cumbered with obsolete machinery.
It is pleaded on St Thomas's behalf that the question before him is a metaphysical one, independent altogether of the manner in which actual species have come into existence.
Of God and His Creatures: 2.41