Reproductive In mammals the female reproductive organs comprise the ova ries, Fallopian tubes or oviducts, uterus and vagina. The ovaries are two in number, a left and a right Approximated to them are the Fallopian tubes, which widen below and form the uterus. In the lower mammals the uterus of each side is separate, but the two unite below in a common vagina, while in the higher groups, the uterus and vagina are both single. The male organs comprise the testes, spermatic cord and penis. In cetaceans, sea-cows and seals, which are aquatic, and in the elephants, conies and many edentates, the testes are inter nal in position, but in most other forms they de scend periodically, or permanently, into a pouch of the integument, called the scrotum. The structure of the penis in mammals is peculiar to the class. An os penis is present in the majority of bats, insectivores, rodents, carniv ores and primates.
During development the mammalian foetus is nourished through a complex structure, called the placenta, formed in part by the internal wall of the uterus of the mother and in part by the membranes of the foetus itself. The placenta is characteristic of the class as a whole, but is not found in the monotremes, nor in most marsupials. The form and other charac teristics of the placenta differ in the several orders of mammals and are regarded as of importance in classification.
geographical distribution of existing mammals, as of other animals, is the result of varied conditions and influences, some transient and others of long continuance, be ginning in the relatively remote geological times when the class first made its appearance. Among the principal factors in the problem of distribution may be included changes in the ex tent and configuration of the land areas of the globe, changes in climate and in food supply, the appearance and disappearance of enemies, and latest, but by no means least, the interfer ence of man. These and other factors in dis tribution are considered under the heading DISTRIBUTION OF LIVING MAMMALS. It iS only possible here to mention some of the more important facts in the distribution of mammals. Of widest distribution are the purely aquatic orders, the cetacea and pinnipedia, whose range covers all seas and reaches from pole to pole, but it should be noted that no sea-lions occur in the north Atlantic. Next follow the bats, whose range is nearly world-wide, but they do not enter the Antarctic zone, and only very few species cross the Arctic Circle. On the other hand, they are found in New Zealand and in oceanic islands where no terrestrial indigenous mammals occur. Of the purely terrestrial or ders, the rodents have the widest range, cover ing every continent and reaching from the Arc tic zone to Patagonia and Tasmania. Carniv
ores, like rodents, have an almost world-wide distribution, but in Australia only one species of the order occurs, the dog known as the Canis dingo; and it is uncertain whether this may not have been introduced by man at a remote date. The monotremes (com prising only the genera Ornithorynchus, Echidna and Proichidna) are limited to Australia, Tas mania and New Guinea. Marsupials occur only in Australia, Tasmania, New Guinea and Amer ica. The American marsupials, with the excep tion of one genus, all belong to the family Didelphiider, or the opossums. Edentates occur only in America, southern Asia and Africa. They have their greatest development in South America. One genus, Tatu, extends northward into Texas. Ungulates inhabit all continents ex cept Australia, but only two or three species enter South America. Of the two groups form ing the order Primates, the lemurs and lemur oids occur only in Madagascar, Africa and southern Asia, while monkeys inhabit only Af rica, southern Asia and South and Central America. One ape, Macacus inuus, is found at Gibraltar, but it is only doubtfully in digenous.
Fossil Mammals.— Mammals are believed to have originated as an offshoot from certain Permian and Triassic reptiles called Theromor pha or Anomodontia. The earliest recognizable remains of mammals are certain small teeth and in the Triassic formations. They belanged to forms resembling monotremes and marsupials in some characters, but are usu ally placed in a separate order, called Allotheria or Multituberculata. Representatives of the Allotheria continued on through the Jurassic and Cretaceous. The existing orders of mam mals first appear in the Eocene, the lowest for mation of the Tertiary period, being foreshad owed in the lowest beds of that period by cer tain generalized groups such as the Creodonta and Condylarthra. The Eocene also contains re mains of several groups, or suborders, of un gulate mammals, which have no living repre sentatives. These are the Ancylopoda, Typothe ria and Toxodontia.
The later Tertiary and the Quaternary pe riods show a greatly increased number and di versity of forms. Many of them represent fam ilies which persisted for only a relatively short period and are now extinct; others have con tinued to the present. Among the oldest of existing genera are Didelphis (opossum), Sciurus (squirrel), Myoxus (dormouse), Sorex (shrew), Vespertilio and Vesperugo (bat) and Viverra (civet), which originated in the Eocene ; Tapirus (tapir) , Rhinoceros, Giraffa (giraffe), Elephas (elephant), Sus (pig), Talpa (mole), Erinaceus (hedgehog), Mustela (mar ten), Liars (otter), Hyena, Felis (cat) and Phoca (seal), which originated in the Miocene.