In the Summa Theologica, First Part, question 73, article 1, St Thomas asks whether the completion of the creatures belongs to the seventh day. In the reply to the third objection he says that “nothing entirely new was afterwards made by God”, and speaks of things existing “in their causes”, of “species” and of “active powers”. The article draws on St Augustine’s teaching on the seminal reasons (rationes seminales), and it stands under this term for that.
Nothing entirely new was afterwards made by God, but all things subsequently made had in a sense been made before in the work of the six days. Some things, indeed, had a previous experience materially, as the rib from the side of Adam out of which God formed Eve; whilst others existed not only in matter but also in their causes, as those individual creatures that are now generated existed in the first of their kind. Species, also, that are new, if any such appear, existed beforehand in various active powers; so that animals, and perhaps even new species of animals, are produced by putrefaction by the power which the stars and elements received at the beginning. Again, animals of new kinds arise occasionally from the connection of individuals belonging to different species, as the mule is the offspring of an ass and a mare; but even these existed previously in their causes, in the works of the six days.
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