88 PRIMARY FACTORS OF ORGANIC EVOLUTION. 



is constructed in the Pelycosauria almost exactly like 

 that of the Prototheria. The single occipital condyle 

 of the reptiles is not found in the Mammalia, but in 

 some of the Lacertilia (Uroplates, Gecco) there are 

 two condyles, the median (basioccipital) portion of the 

 single condyle being rudimental, and Seeley has re- 

 cently shown that it is deeply divided at the middle in 

 the Permian Cynognathidae of South Africa.. The 

 Pelycosauria could not, however, have given origin to 

 the Prototheria, since in that subclass of mammals there 

 is a well-developed coracoid. But in the Procolopho- 

 nina this element is developed as in the Prototheria. 

 Moreover, the Pelycosauria and the Procolophonina 

 have the interclavicle, which is an element of membran- 

 ous origin, while in the Prototheria we have the corre- 

 sponding cartilage bone, the episternum. This element 

 is' present in the Permian order of the Cotylosauria, 

 which is nearly related to the Pelycosauria. This or- 

 der has, however, single-headed ribs, springing from 

 the diapophyses, which is not usual in the Mammalia. 

 But in some Cotylosauria the diapophyses are short, 

 and in the Monotremata the postcervical ribs are sin- 

 gle-headed, so this character is not an insurmountable 

 one. It is evident that the Mammalia were derived 

 from some type probably referable to a Permian rep- 

 tilian order of the Theromorous series, although to 

 which one is not yet known. 



The Reptilia have been supposed by Haeckel to 

 have taken their origin from the Batrachia. I have 

 indicated that it is probable that the batrachian order, 

 which stands in this relation to the Reptilia, is the 

 Embolomeri of the Permian epoch. This conclusion 

 rests on the following considerations. The reptilian 

 order of the Cotylosauria approaches the Batrachia of 



