"…a particular provider excludes all defects from what is subject to his care as far as he can; whereas, one [like God] who provides universally allows some little defect to remain, lest the good of the whole should be hindered. Hence, corruption and defects in natural things are said to be contrary to some particular nature; yet they are in keeping with the plan of universal nature; inasmuch as the defect in one thing yields to the good of another, or even to the universal good: for the corruption of one is the generation of another, and through this it is that a species is kept in existence. Since God, then, provides universally for all being, it belongs to His providence to permit certain defects in particular effects, that the perfect good of the universe may not be hindered, for if all evil were prevented, much good would be absent from the universe."
«Quia provisor particularis excludit defectum ab eo quod eius curae subditur, quantum potest, sed provisor universalis permittit aliquem defectum in aliquo particulari accidere, ne impediatur bonum totius. Unde corruptiones et defectus in rebus naturalibus, dicuntur esse contra naturam particularem; sed tamen sunt de intentione naturae universalis, inquantum defectus unius cedit in bonum alterius, vel etiam totius universi; nam corruptio unius est generatio alterius, per quam species conservatur. Cum igitur Deus sit universalis provisor totius entis, ad ipsius providentiam pertinet ut permittat quosdam defectus esse in aliquibus particularibus rebus, ne impediatur bonum universi perfectum.»
Read Michael Liccione's essays on "The Problems of Evil" and "Mystery and Explanation in Aquinas' Account of Creation".