Objection 1. It would seem that natural contingents are not subject to the eternal law. Because promulgation is essential to law, as stated above (Question 90, Article 4). But a law cannot be promulgated except to rational creatures, to whom it is possible to make an announcement. Therefore none but rational creatures are subject to the eternal law; and consequently natural contingents are not.
Objection 2. Further, "Whatever obeys reason partakes somewhat of reason," as stated in Ethic. i. But the eternal law, is the supreme type, as stated above (Article 1). Since then natural contingents do not partake of reason in any way, but are altogether void of reason, it seems that they are not subject to the eternal law.
Objection 3. Further, the eternal law is most efficient. But in natural contingents defects occur. Therefore they are not subject to the eternal law.
On the contrary, It is written (Proverbs 8:29): "When He compassed the sea with its bounds, and set a law to the waters, that they should not pass their limits."
I answer that, We must speak otherwise of the law of man, than of the eternal law which is the law of God. For the law of man extends only to rational creatures subject to man. The reason of this is because law directs the actions of those that are subject to the government of someone: wherefore, properly speaking, none imposes a law on his own actions. Now whatever is done regarding the use of irrational things subject to man, is done by the act of man himself moving those things, for these irrational creatures do not move themselves, but are moved by others, as stated above (Question 1, Article 2). Consequently man cannot impose laws on irrational beings, however much they may be subject to him. But he can impose laws on rational beings subject to him, in so far as by his command or pronouncement of any kind, he imprints on their minds a rule which is a principle of action.
Now just as man, by such pronouncement, impresses a kind of inward principle of action on the man that is subject to him, so God imprints on the whole of nature the principles of its proper actions. And so, in this way, God is said to command the whole of nature, according to Psalm 148:6: "He hath made a decree, and it shall not pass away." And thus all actions and movements of the whole of nature are subject to the eternal law. Consequently irrational creatures are subject to the eternal law, through being moved by Divine providence; but not, as rational creatures are, through understanding the Divine commandment.
Reply to Objection 1. The impression of an inward active principle is to natural things, what the promulgation of law is to men: because law, by being promulgated, imprints on man a directive principle of human actions, as stated above.
Reply to Objection 2. Irrational creatures neither partake of nor are obedient to human reason: whereas they do partake of the Divine Reason by obeying it; because the power of Divine Reason extends over more things than human reason does. And as the members of the human body are moved at the command of reason, and yet do not partake of reason, since they have no apprehension subordinate to reason; so too irrational creatures are moved by God, without, on that account, being rational.
Reply to Objection 3. Although the defects which occur in natural things are outside the order of particular causes, they are not outside the order of universal causes, especially of the First Cause, i.e. God, from Whose providence nothing can escape, as stated in the I, 22, 2. And since the eternal law is the type of Divine providence, as stated above (Article 1), hence the defects of natural things are subject to the eternal law.
Source: Summa Theologica (New Advent)