132 PRIMARY FACTORS CF ORGANIC EVOLUTION. 



The characters of the skeleton of the order Mono- 

 tremata show that it is nearest of kin to the Reptilia, and 

 many subordinate characters, especially of the extrem- 

 ities, point to the Theromora as its ancestral source. 1 

 In the general characters the Marsupialia naturally 

 follow in a rising scale, as proved by the increasing 

 perfection of the reproductive system. The Monodel- 

 phia follow with improvements in the reproductive 

 system and the brain, as indicated in the table above 

 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 Condylar- 

 thra and Amblypoda of extinct groups, and the Buno- 

 theria, Edentata, Glires, and Chiroptera, recent and 

 extinct. The characters of the brains of Amblypoda 

 and some Creodonta are, in their superficial char- 

 acters, even inferior to existing marsupials. The di- 

 vided uterus of the recent forms named, also gives 

 them the position next to the Marsupialia. In the 

 Carnivora, Hyracoidea, and Proboscidia, a decided ad- 

 vance in both brain-structure and reproductive system 

 is evident. The hemispheres increase in size, and they 

 become convoluted. A uterus is formed, and the testes 

 become external, etc. In the Quadrumana and An- 

 thropomorpha the culmination in these parts of the 

 structure is reached, excepting only that, in the lack 

 of separation of the genital and urinary efferent ducts, 

 the males are inferior to those of many of the Artio- 

 dactyla. This history displays a rising scale for the 

 Mammalia. 2 



1 Proceedings American Philosoph. Society, 1884, p. 43. Antea, p. 87. 



2 See the evidence for evolution in the history of the extinct Mammalia. 

 Proceedings of 'the American Association for the Advancement of Science, 1883. 



