from injury, from dehydration, and from fungal and bacterial attack. The constant thickness of the cornified layer results most commonly from regular superficial exfoliation. When a specialised cornified structure has a particular toughness, as in nails and hair, little or no exfoliation occurs, and the cornified structures must slowly migrate away from their original position. Thus, the specialised cornified structures of the hoof are the wall, the sole, the frog and the periople. The wall does not exfoliate at all; it is constantly growing downward (about 1 cm per month), and self-trims by wearing or chipping by ground contact. In wild and feral horses, solar, frog and periople materials grow outwards and exfoliate at the surface by ground contact and wearing. In the domesticated horse, movement and typical ground hardness are insufficient to allow self-trimming, so humans have to care for them by trimming the walls and the frog, and scraping off the dead sole. Hoof changes over the horse's lifetime The front and hind hooves are identical in the foal but differ visibly in the adult horse. This is good evidence of the medium-term plasticity of the whole hoof shape, as a result of variation in its use. Slow changes in hoof shape occur under any consistent change in the horse's movement pattern and under a wide variety of pathological conditions. They can be seen now as a clear example of a complex adaptive system, a frequent feature of living beings and structures. Self-adapting capabilities of the hooves show their maximal effectiveness in wild equids (but domesticated horses show this too, to a lesser extent), as shown by the perfect soundness of feral horses, such as Mustangs, in a wide variety of environments. Hoof evolution Equid hooves are the result of the 55-million-year evolution of the horse. The ancestral horse, Eohippus, is characterized by four toes on the hindfeet and three toes on the forefeet. Wild and domesticated Equus species share a very similar hoof shape and function. The present-day conformation of the hoof is a result of a progressive evolutionary loss of digits I, II, IV and V of the basal pentadactyl limb, with changes in bones, joints, and the hoof capsule. The resulting conformation allows a heavy, strong body to move with high speed on any ground, and most efficiently on open, hard, flat areas like prairies and deserts (an example of cursorial specialisation). A 2018 study