262 THE DOCTRINE OF DESCENT. 
differentiation of structure is to be observed, as in the 
Lacertilia, or Crocodilia, it goes no further than a modi- 
fication of the form of the articular surfaces of the verte- 
bra, or of the degree to which the internal nasal aper- 
tures are surrounded by bone. The osteological differ- 
ences, which alone are exhibited by fossil remains, have 
doubtless been accompanied by many changes in the 
organization of the destructible parts of the body; but 
everything tends to show that the amount of change in 
the organization of reptiles since their first known ap- 
pearance upon the earth, is not great in itself; and is 
wholly insignificant, if we take into consideration the 
lapse of time, and the changes of the surface of the 
globe, which are represented by the Mesozoic and Ter- 
tiary formations. 
“From the point of view of the evolution hypothesis, 
it is necessary to suppose that the Reptilia have all 
sprung from a common stock, and I see no justification 
for the supposition that the rapidity of their divergence 
from this stock was greater before the epoch of the 
Trias than it has been since. Consequently, seeing 
that the approximation of the oldest known representa- 
tives of the different orders is so slight, reptiles must 
have lived before the Trias for a length of time, com- 
pared with which that which has elapsed from the 
Triassic epoch until now is small—in other words, the 
commencement of the existence of reptiles must be 
sought in a remote palzozoic epoch.” 
Comparison thus points us back to ages which afford 
no record of the actual derivation of this class. . Even 
the Ichthyosauria and Plesiosauria, so frequently men- 
tioned in conjunction, deviate widely from one another 
