2. Varieties may originate in two different ways. In the case of wild animals it is generally supposed that they are gradually induced by the slow operation of external influences; but it is certain that in domesticated animals they often appear suddenly and unexpectedly, and are not on that account at all less permanent. A large proportion of our breeds of domestic animals appear to originate in this way. A very remarkable instance is that of the "Niata" cattle of the Banda Orientale, described by Darwin in his "Voyage of a Naturalist." These cattle are believed to have originated about a century ago among the Indians to the south of the La Plata, and the breed propagates itself with great constancy. "They appear," says Darwin, "externally to hold nearly the same relation to other cattle which bull-dogs hold to other dogs. Their forehead is very short and broad, with the nasal end turned up, and the upper lip much drawn back; their lower jaws project outward; when walking they carry their heads low on a short neck, and their hinder legs are rather longer compared with the front legs than is usual." It is farther remarkable in respect to this breed that it is, from its conformation of head, less adapted to the severe droughts of those regions than the ordinary cattle, and can not, therefore, be regarded as an adaptation to circumstances. In his later work on animals under domestication, Darwin gives many other instances of the origination of breeds of cattle and other animals in this abrupt and mysterious manner, and without any selection, though he strongly leans to the conclusion that slow and gradual changes are the most frequent causes of variation. It is to be observed, however, that very slow changes are in more danger of being accidentally diverted or obliterated by crossing, and that the first stages of an incipient change may be too unimportant to be permanent. Many writers on the subject of the Unity of Man assume that any marked variety must require a long time for its production. Our experience in the case of the domestic animals teaches the reverse of this view; a very important point too often overlooked.