SURVEY OF MAMMALS. £25 



(^.) The duckmole {Ornithorhynchus) and the porcupine 

 ant-eater {Echidna) differ from all other Mammals in many ways. 

 The young are hatched outside of the body ; in other words, 

 the mothers are oviparous. The brain seems but poorly devel- 

 oped when compared with that of other Mammals. Some of 

 the characteristics of the skeleton and other structures sug- 

 gest Reptilian affinities. To this small sub-class, the titles 

 Monotremata, Ornithodelphia, and Prototheria are applied. 



(B^ The kangaroos and bandicoots, phalangers and opos- 

 sums, and the like, form the second sub-class, the members 

 of which are called Marsupials. In these the young are 

 born prematurely, after a short gestation during which the 

 organic connection between the mothers and the young 

 is comparatively slight. Most female Marsupials have an 

 external pouch or marsupium, to which the tender young 

 are transferred, and within which they are nourished and 

 protected for some time. Moreover, the brains even of the 

 most intelligent Marsupials are not so well developed as 

 those of higher Mammals. To this sub-class, the titles 

 Marsupialia, Didelphia, and Metatheria are applied. 



(C.) In all the other Mammals there is a close connec- 

 tion or placenta vitally uniting the unborn young to the 

 mother. It is among these placental Mammals that the 

 brains begin to be much convoluted, — as it were, wrinkled 

 with thought. To this sub-class, including sloths and ant- 

 eaters (Edentata), sea-cows (Sirenia), hoofed-animals (Ungu- 

 lata), Cetaceans, Rodents, Carnivores, Insectivores, Bats, 

 Lemurs, and Monkeys, the titles Placentalia, Monodelphia, 

 and Eutheria are applied. 



Let us try to take a bird's-eye view of the whole series, 

 and especially of the numerous orders of placental Mam- 

 mals. Among these it seems likely that the Edentata 

 and Sirenia should be placed lowest, for many of their 

 characteristics are distinctly old-fashioned. The rest may 

 be provisionally grouped in three sets, perhaps representing 

 the three main lines of evolution. 



On one side let us place the great series of hoofed animals 

 or Ungulata, including (a) those with an even number of 

 toes (Artiodactyla), such as pigs, hippopotamus, camels, 

 cattle, and deer; (V) those with an odd number of toes 

 (Perissodactyla), such as tapir, rhinoceros, and horse ; (c) the 



