Besides the ruminating artiodactyles there is also an extensive group of non-ruminating artiodactyles, made up of all the various kinds of swine (including the American peccaries), together with the hippopotamus, now found nowhere but in Africa. Distinct as are the ruminating and non-ruminating artiodactyles now, they were in ancient time connected by a great number of intermediate forms which have utterly passed away.
The llamas of South America represent the camels of the Old World, where the latter are to-day exclusively found. When South America was discovered by the Spaniards, llamas were the only beasts of burthen found there, and, indeed, the only cattle of any kind then and there existing; although horses had formerly abounded and had become extinct in South America at a long anterior period.
Somewhat allied to ungulates, but distinct from them, are the elephants, which form an order (Proboscidea) by themselves—an order once rich in many species widely distributed over the earth.
Hardly less familiar than our domestic animals, are our hares, rabbits, mice, squirrels, and their allies, which together form an "order" called Rodentia from the gnawing habits of its members which nourish themselves on vegetable substances. This order of rodents is very rich in species, and consists of many genera grouped in several distinct families—such, e.g., as the family of mice and rats (Muridæ), of squirrels (Sciuridæ), of guinea-pigs and spine-bearing porcupines (Hystricidæ), &c. The largest form of rodent is the capybara (or river-hog of the Rio de la Plata),—which is preyed on by the jaguar. Though a near ally of the little guinea-pig, it is as large as a hog. Amongst the more interesting rodents may be mentioned beavers,[11] the fur-bearing chinchilla, the jerboa (Dipus), the musk-rat (Fiber), and the rat-mole (Spalax). The jerboa has very long hind legs, and a habit of jumping, so that it resembles superficially (but not really) a small kangaroo. The Spalax is quite blind, and has the burrowing habit, and somewhat the shape of the common mole. Some rodents are fitted to flit through the air in long jumps, by means of the wide extensibility of the skin of their flanks, which, when stretched out, acts as a parachute. Such forms are the flying squirrels, and a curious rodent called Anomalurus, from the exceptional clothing of the base of its tail, which is furnished with large scales at its under part.
Another order of beasts may here be referred to, because it affords interesting examples of the co-existence of external resemblance without any real affinity. This order includes the insect-eating beasts, or Insectivora, and comprises the moles, hedgehogs, shrew-mice (which are not really "mice" at all), and their allies. The Insectivora and Rodentia present us with a singular parallelism in the respective modifications of structure, which are found in these two very distinct orders. But the insectivorous forms (as might perhaps be expected from their less abundant food) are always smaller in size than are the parallel vegetable-eating groups of rodents. Indeed, one insectivore of the genus Sorex (the shrew-mouse genus) is the absolutely smallest mammal which is known to exist.
As examples of the parallelism referred to may be mentioned the moles (which resemble the rat-moles), the shrew-mice (which resemble true mice), the hedgehogs, and the less known spiny tanrec of Madagascar (which resemble porcupines in their clothing); certain graceful and active tree-frequenting insectivores of the Indian Archipelago, Tupaia (which resemble squirrels); an aquatic African form, Potomogale (which resembles the musk-rat); certain elephant shrews—long-legged, jumping, African insectivores (which resemble the jerboa amongst rodents); and, lastly, the so-called flying lemur of the Philippine Islands, or Galeopithecus, which resembles the flying squirrel, and the curious rodent Anomalurus before referred to.
The only beasts, however, which truly fly are the bats, which form an order by themselves, well-named, from the structure of their wings, Cheiroptera. The bats which fly about in the twilight in this country, or sometimes in the afternoon of a warm day in winter, are all insect-eating forms. But in the warm regions of the Old World, and of Australia, there are large fruit-eating kinds, called "flying foxes;" while in South America there are blood-sucking bats, or vampires, some of which, as we shall hereafter see, present the most curious and interesting modifications of structure in harmony with their peculiar habits.
The creatures which are in some respects the most interesting to us, because they are the most like ourselves in form, are the apes. Moreover, not only are they so like us in form, but they are so widely marked-off from all other creatures except ourselves, that it seems impossible they can have any real affinity to one more than to another group of mammals below man. Apes and man then together form one order, which as ranking first was named by Linnæus, Primates. With the apes are commonly associated certain animals called Lemurs, which inhabit the vicinity of the Indian Ocean, especially Madagascar. They have not, however, any real affinity to apes; and if they are to be placed in the same order at all, they must be well distinguished from its other members. It has therefore been proposed[12] to divide the order Primates into two sub-orders (as the hoofed order is divided into the "odd-toed" and "even-toed" sub-orders), one of these to include man and apes, and to be called, from the resemblance to the human form pervading it, "Anthropoidea;" the other sub-order to be termed "Lemuroidea."
The first "sub-order" is divisible into three "families." One of these (Hominidæ) contains man (forming the genus Homo), the second (Simiadæ) contains all the apes of the Old World only, while a third (Cebidæ) contains all those of America.
Amongst the Simiadæ are the orang, the chimpanzee, the gorilla, and the long-armed apes (or Gibbons), which are the most man-like of all the apes; and there can be no question but that there is very much less difference in structure between these four kinds of apes and man, than there is between them and the lowest of the apes—i.e., the marmosets.