ORIGIN OF PLACENTAL MAMMALS. 247 



of a decidua, but also in the external form of the placenta 

 itself. In the Indeciduata it consists, in most cases, of 

 numerous, single, scattered bunches or tufts of vessels, and 

 hence this group may be called tufted 'placental animals, 

 (Villiplacentalia). In the Deciduata, however, the single 

 tufts of vessels are united into a cake, which appears in two 

 different forms. In the one case it surrounds the embryo in 

 the form of a closed band or ring, so that only the two poles 

 of the oval egg bladder are free of tufts ; this is the case in 

 animals of prey (Carnaria) and the pseudo-hoofed animals 

 (Chelophora), which may consequently be comprised as 

 girdled-placental animals (ZonoplacentaHa). In the other 

 Deciduata, to which man also belongs, the placenta is a 

 simple round disc, and we therefore caU them disc-placen- 

 tals (Discoplacentalia). This group includes the five orders 

 of Semi-apes, Gnawing animals, Insectivora, Bats, and Apes, 

 from the latter of which, in the zoological system, man 

 cannot be separated. 



It may be considered as quite certain, from reasons based 

 upon their comparative anatomy and their history of de- 

 velopment, that Placental animals first developed out of 

 Marsupials, and that this very important development — the 

 first origin of the placenta — probably took place in the 

 beginning of the tertiary epoch, during the eocene period. 

 But one of the most difficult questions in the genealogy of 

 animals is the important consideration whether all Placental 

 animals have arisen out of one or out of several distinct 

 V)ranches of Marsupials ; in other words, whether the origin 

 of the placenta occurred but once, or several times. 



When, in my General Morphology, I for the first time 

 endeavoured to establish the pedigree of Mammals, I here, 



