344 THE STRUCTURAL EVIDENCE OF EVOLUTIO:^. 



characters point to the Theromorpha as its ancestral source.* In 

 the general characters the Marsupialia naturally follow in a rising 

 scale, as proved by the increasing perfection of the reproductive 

 system. The Monodelphia follow with improvements in the re- 

 productive system and the brain, as indicated in the table already 

 given. The oldest Monodelphia were, in respect to the structure 

 of the brain, much like the Marsupialia, and some of the existing 

 orders resemble them in some parts of their brain-structure. 

 Such are the Condylarthra and Amblypoda of extinct groups, 

 and the Bunotheria, Edentata, Rodentia, and Chiroptera, recent 

 and extinct. The characters of the brains of Amblyj^oda and some 

 Creodonta are, in their superficial characters, even inferior to ex- 

 isting marsupials. The divided uterus of the recent forms named, 

 also gives them the position next to the Marsupialia. In the Gar- 

 ni vora, Hyracoidea, and Proboscidia, a decided advance in both 

 brain-structure and reproductive system is evident. The hemi- 

 spheres increase in size, and they become convoluted. A uterus 

 is formed, and the testes become external, etc. In the Quadru- 

 mana the culmination in these parts of tlie structure is reached, 

 excepting only that, in the lacic of separation of the genital and 

 urinary efferent ducts, the males are inferior to those of many of 

 the Artiodactyla. This history displays a rising scale for the 

 Mammalia. 



Looking at the skeleton, we observe the following successional 

 modifications : f 



First, as to the feet, and (A) the digits. The Condylarthra 

 Iiave five digits on both feet, and they are plantigrade. This char- 

 acter is retained in their descendants of the lines of Anthropoidea, 

 Quadrumana, and Hyracoidea, also in the Bunotheria, Edentata, 

 and most of the Rodentia. In the Amblypoda and Proboscidia 

 the palm and heel are a little raised. In the Carnivora and Dip- 

 larthra the heel is raised, often very high, above the ground, and 

 the number of toes is diminished, as is well known, to two in the 

 Artiodactyla and one in the Perissodactyla. (B) The tarsus and 

 carpus. In the Condylarthra the bones of the two series in the 

 carpus and tarsus are opposite each other, so as to form continuous 

 and separate longitudinal series of bones. This continues to be 



* "Proceeding3 American Philosoph. Society," 1884, p. 43. 

 f Sec the evidence for evolution in the history of the extinct Mammalia. " Pro- 

 ceeds. Araer. Assoc. Adv. Science," 1883. 



