CHAP. Ill] THE MAMMALIAN ORDER PRIMATES 41 



consideration after the more natural representation provided in Fig. 15, where 

 the really tortuous nature of the several lines is sufficiently evident. The 

 doctrine of " evolution from the most specialized" is so important in regard 

 to the Phylogeny of Man, that its notification here is absolutely necessary, 

 and it will be discussed further in the sequel. For the moment, the diagram 

 .submitted in Fig. 15 is to be regarded not as expressing all the facts of the 

 case, but as the most convenient form of expression available 1 . 



The diagnostic characters of the Prototheria, given in Chapter II 

 (p. 26), provide evidence of the lowly position assigned to those 

 animals in relation to the other Mammalia. Some of the more 

 important items are given in the following list. 



(1) The possession of oviducts which are distinct from each 

 other throughout their course. 



(2) The possession of a cloaca. 



(3) The lack of vertebral epiphysial plates. 



(4) The structure of the shoulder girdle (full development of 

 the several elements). 



(5) The production of meroblastic ova, 



as well as the physiological characteristic of being oviparous. 



Turning now to the Metatheria (Marsupialia), the evidence for 

 the lowlier status of these as compared with the Eutherian 

 mammals depends chiefly on the conformation of the brain, which 

 lacks the great cerebral commissure known as the corpus cal- 

 losum; herein the brains of Metatheria (Marsupialia) resemble 

 those of Monotremata and Reptilia, while differing from those 

 of Eutheria. The mode of development of the embryo of some 

 Metatheria provides additional evidence of the highest significance, 

 viz. the relatively large size of the ovum and its enclosure by 

 a shell membrane, discovered in the Marsupial Dasyurus by Pro- 

 fessor J. P. Hill 2 . These characters place the genus (Dasyurus) in 

 a lowly position among the Metatheria. The genus Perameles 

 also possesses primitive characters as compared with its associates. 



Within the range of the Eutheria, the primitive characters 

 (notably those of the brain) presented by the Insectivora have the 



1 For useful additional data in regard to such genealogical trees, v. Dendy, 

 Outlines of Evolutionary Biology, Figs. 85 and 86. 



- v. Nature, Oct. 22, 1908, p. 649, and the fuller account in Q. J. M. S. 1910. 



