Adam Clarke on Genesis 30:38

And he set the rods which he had pilled before the flocks - It has long been an opinion that whatever makes a strong impression on the mind of a female in the time of conception and gestation, will have a corresponding influence on the mind or body of the fetus. This opinion is not yet rationally accounted for. It is not necessary to look for a miracle here; for though the fact has not been accounted for, it is nevertheless sufficiently plain that the effect does not exceed the powers of nature; and I have no doubt that the same modes of trial used by Jacob would produce the same results in similar cases. The finger of God works in nature myriads of ways unknown to us; we see effects without end, of which no rational cause can be assigned; it has pleased God to work thus and thus, and this is all that we know; and God mercifully hides the operations of his power from man in a variety of eases, that he may hide pride from him. Even with the little we know, how apt are we to be puffed up! We must adore God in a reverential silence on such subjects as these, confess our ignorance, and acknowledge that nature is the instrument by which he chooses to work, and that he performs all things according to the counsel of his own will, which is always infinitely wise and infinitely good.