absence in its brain of signs of serial elevation; and it exhibits a great dissimilarity to all, even the lowest of the Old World forms, and to those American Monkeys, which in brain-character closely resemble Old World Apes. It shows an affinity in some of its more striking characters to such low forms of New World Apes as _Nyctipithecus_. The low type of brain is in keeping, as Sir William Flower further observes, with their surly and untameable disposition, and with the observation that their intelligence is of a very different order from that of their neighbours, the Spider-Monkeys and Capuchins of higher cerebral organisation. "When Howlers are seen in the forest," remarks Mr. Bates, "there are generally three or four of them mounted on the topmost branches of a tree. It does not appear that their harrowing roar is emitted from sudden alarm; at least, it was not so in captive individuals. It is probable, however, that the noise serves to intimidate their enemies." The muscular power employed in giving vent to their cavernous roar appears to be small. Their food consists chiefly of fruits and leaves. In colour the Howlers vary very much. The young of both sexes often differ from their parents, and the females from the males, and there is also great individual variation. The geographical distribution of some of the species is very restricted, several of them being confined to a special district {192}of the Amazon, into which no other species intrudes. They are found, however, from Eastern Guatemala to Paraguay. THE HOWLERS. GENUS ALOUATTA. _Alouatta_, Lacép., Mém. Inst., iii., p. 490 (1801). _Mycetes_, Illig., Prodr. Syst. Mamm., p. 70 (1811). _Stentor_, Geoffr., Ann. Mus., xix., p. 107 (1812). The characters of the genus _Mycetes_, which is the only one of the sub-family, are the same as those given above under the sub-family heading. The genus contains six well-recognised species. According to Mr. Wallace the red and black species of the Amazon have females of the same colour as the males. Humboldt also remarks, speaking of the thousands of Arguatoes (_M. seniculus_) which he observed in the provinces of Cumana, Caracas, and in Guiana, that he never saw any change in the reddish-brown fur of the back and shoulders, either in isolated individuals or whole troops. Many of the species, however, do have the sexes of quite different colours. The Howlers are semi-nocturnal in their habits, uttering their cries late in the evening and before sunrise, and also on the approach of rain. (_Wallace._) When a _Mycetes_ is shot it always hangs to the tree, even if quite dead, and does not fall till the muscles of the feet and tail relax. The species of this genus range through Central America, Colombia, and the Amazonian region, to Southern Brazil, Bolivia, and Paraguay. PLATE XVIII. [Illustration: THE RED HOWLER.] {193}I. THE RED HOWLER. ALOUATTA SENICULUS. _Simia seniculus_, Linn., Syst. Nat., i., p. 37 (1766).