138 PRIMARY FACTORS OF ORGANIC EVOLUTION. 



become multiplied so as to produce numerous cross- 

 crests. 



The molars of some of the Sirenia are like that of 

 some of the Ungulata, especially of the tapirine group, 

 while in others the teeth consist of cylinders. In the 

 Cetacea the molars of the oldest (Eocene and Miocene) 

 types are but two-rooted and compressed, having much 

 the form of the premolars of other Mammalia. In ex- 

 isting forms a few have simple conical teeth, while in 

 a considerable number teeth are entirely wanting. 



g. General Review of the Phytogeny of Mammalia. 



In the accompanying table some of the characters 

 of the mammalian skeleton above described are thrown 

 into a tabular form. They are exhibited in the order 

 of their appearance in geological time, beginning with 

 the oldest horizon at the bottom of the left-hand col- 

 umn. Continued primitive types are enclosed in brack- 

 ets. These relations were pointed out by me in 1883, ' 

 and every discovery made since that date has confirmed 

 their correctness. Some characters of the Mesozoic 

 Mammalia are now added. 



Paleontology has cleared up the phylogeny of most 

 of the mammalian orders, but some of them remain as 

 yet unexplained. This is the case with the Cetacea, 

 the Sirenia, and the Edentata. The Marsupialia can 

 be supposed with much probability to have come off 

 from the Monotremata, but there is but little paleon- 

 tological evidence to sustain the hypothesis. Little 

 progress has been made in unravelling the phylogeny 



"^Proceedings American Assoc, Adv. Science, p. 40. The successional gra- 

 dation in the limbs and teeth was announced by me in 1873 {Proceeds. Acad- 

 emy Philadelphia, p. 371, and Journal of the Academy, 1874, p. 20), and that in 

 the size of the hemispheres of the brain by Marsh in 1874 (American Journal 

 Sci, Arts, p, 66). 



