n6 PRIMARY FACTORS OF ORGANIC EVOLUTION. 



The diagrams on pages 117-119 illustrate the suc- 

 cessive changes in the structure of the posterior region 

 of the skull in the types mentioned. The orders and 

 suborders Pseudosuchia, Rhynchocephalia, Ichthyo- 

 pterygia, Dinosauria, Crocodilia, Sauropterygia, and 

 Testudinata commence at the beginning of Mesozoic 

 time, after the Permian had closed. The Squamata 

 (lizards and snakes) commence, so far as is certainly 

 known, in the later Mesozoic, in the Cretaceous period. 



The line which terminated in the Lacertilia and. 

 Ophidia (Squamata) may have originated directly from 

 the Theriodonta, or it may have descended from the 

 Rhynchocephalia. It departs from the former type in 

 two respects : 



First, in the loss of the capitular articulation of the 

 ribs, and, second, in the gradual elongation and final 

 freedom of the suspensory bone of the lower jaw (the 

 os quadratum). In so departing from the Theromora, 

 it also departs from the mammalian type. The ribs 

 assume the less perfect kind of attachment which the 

 mammals only exhibit in some of the whales, and the 

 articulation of the lower jaw loses in strength, while 

 it gains in extensibility, as is seen in the develop- 

 ment of the line of the eels among fishes. The end 

 of this series, the snakes, must therefore be said to be 

 the result of a process of creation by degeneration, and 

 their lack of scapular arch and fore limb and usual 

 lack of pelvic arch and hind limb are confirmatory 

 evidence of the truth of this view of the case. 



Secondly, as regards the ossification of the anterior 

 part of the brain-case. This is deficient in some of 

 the Theromora, the ancestral series, which resemble in 

 this, as in many other things, the contemporary Ba- 

 trachia. The late orders mostly have the anterior walls 



