“The power of imagination upon our physical condition, even after we arrive at maturity, is evinced in many familiar ways. In medicine, the intelligent physician does not hesitate to accord to it a curative or morbific potency greater than his pills and potions. He calls it the vis medicatrix naturae, and his first endeavor is to gain the confidence of his patient so completely, that he can cause nature to extirpate the disease.
Fear often kills; and grief has such a power over the subtile fluids of the body as not only to derange the internal organs but even to turn the hair white. Ficinus mentions the signature of the foetus with the marks of cherries and various fruits, colors, hairs, and excrescences, and acknowledges that the imagination of the mother may transform it into a resemblance of an ape, pig, or dog, or any such animal.
Marcus Damascenus tells of a girl covered with hair and, like our modern Julia Pastrana, furnished with a full beard; Gulielmus Paradinus, of a child whose skin and nails resembled those of a bear; Balduinus Ronsaeus of one born with a turkey’s wattles; Pareus, of one with a head like a frog; and Avicenna, of chickens with hawks’ heads.
In this latter case, which perfectly exemplifies the power of the same imagination in animals, the embryo must have been stamped at the instant of conception when the hen’s imagination saw a hawk either in fact or in fancy.
This is evident, for Dr. More, who quotes this case on the authority of Avicenna, remarks very appropriately that, as the egg in question might have been hatched a hundred miles distant from the hen, the microscopic picture of the hawk impressed upon the embryo must have enlarged and perfected itself with the growth of the chicken quite independently of any subsequent influence from the hen.”
H. P. Blavatsky
The Mark Of God: Understanding Your "I AM Presence" 