It will be well to summarize some of the main facts regarding another and important environmental feature of the Saharan plants, namely, the relation to herbivorous animals. In the western Sahara, wherever there is any forage, animals which subsist on it are to be found. Of the wild animals the gazelle is probably the most numerous and the most destructive. All travelers across the desert have noted the presence of this animal. In the northern Sahara Tristram remarked its abundance fifty or more years ago, and it may be frequently seen by the traveler at the present time. In addition to the native animals, the domestic animals, especially the sheep, goats, and camels, are very numerous, very destructive of plants, and range great distances for food. As a result, an area around every well or oasis, extending as far as 40 kilometers or even much more than this,[38] is repeatedly grazed over and has been utilized in this manner for centuries. As a result only the poisonous or the distasteful species, or the plants especially well armed, are left undisturbed to grow and reproduce, while the balance are more or less consumed, frequently so much so as to be quite unrecognizable. From the large number of camels, sheep, and goats which range the desert pastures it might be concluded that the leading types of plants to be found would be such as are not eaten by them, but this is not the case. On the other hand, possibly the most generally consumed form is _Haloxylon_, which grows on the oued flood-plain, the reg, and the hamada. Thus, so far as this type is concerned, there is probably little or no diminution in numbers because of the attacks of animals. A similar conclusion would doubtless be drawn after study of other forms, but there lies at least one notable exception, namely, the influence of animal grazing on the distribution of the betoum (_Pistacia atlantica_). The betoum, which is the largest arboreal species in the Sahara, is confined to the region of the Dayas; that is, to the country immediately south of Laghouat. The tree is unarmed and is eagerly sought after by all herbivorous animals for its foliage and tender twigs. Owing to the presence of such animals, wild and domesticated, the young tree would have no chance to survive were it not that, growing in association with it, is the jujube (_Zizyphus lotus_), which is armed and is not eaten by any animals. The jujube affords safe protection for the seedling betoum, and in its capacity as nurse prevents predatory attacks by animals during the critical period. The survival (and probably the distribution as well) of the betoum is mainly conditioned on the presence of its protector.