32 THE EIDDLE OF THE UNIVERSE. 



years ago ! In all these important characteristics 

 man is a true mammal. 



The numerous orders (12-33) which modern syste- 

 matic zoology distinguishes in the class of mammals 

 had been arranged in 1816 (by Blainville) in three 

 natural groups, which still hold good as sub-classes : 

 (1) the monotrema, (2) the marsupialia, and (3) the 

 placentalia. These three sub-classes not only differ 

 in the important respect of bodily structure and 

 development, but they correspond, also, to three 

 different historical stages in the formation of the 

 class, as we shall see later on. The monotremes of 

 the Triassic period were followed by the marsupials of 

 the Jurassic, and these by the placentals of the 

 Cretaceous. Man belongs to this, the youngest, sub- 

 class ; for he presents in his organisation all the 

 features which distinguish the placentals from the 

 marsupials, and the still older monotremes. First of 

 all there is the peculiar organ which gives a name to 

 the placentals — the placenta. It serves the purpose 

 of nourishing the young mammal embryo for a long 

 time during its enclosure in the mother's womb ; it 

 consists of blood-bearing tufts which grow out of the 

 chorion surrounding the embryo, and penetrate cor- 

 responding cavities in the mucous membrane of the 

 maternal uterus ; the delicate skin between the two struc- 

 tures is so attenuated in this spot that the nutriment 

 in the mother's blood can pass directly into the blood 

 of the child. This excellent contrivance for nourishing 

 the embryo, which makes its first appearance at a 

 somewhat late date, gives the foetus the opportunity of 

 a longer maintenance and a higher development in 

 the protecting womb ; it is wanting in the implacen- 

 talia, the two older sub-classes of the marsupials and 



