DE GENERA TION. 



223 



things at the close of the Cretaceous age, and exhibiting little or no 

 change during their relatively brief history. 



Such cases of stability amid conditions which might well have fa- 

 vored change, and which saw^ copious modification and progression in 

 other groups of animals, might at first sight be regarded as presenting 

 a serious obstacle to the doctrine of progressive development on which 

 the whole theory of evolution depends. As such an obstacle, the se- 

 ries of facts in question was long regarded ; as such, these facts are 

 sometimes even now advanced, but only by those who imperfectly ap- 

 preciate and only partially understand what the doctrine of evolution 

 teaches and what its leading idea includes. Even Cuvier himself, when 

 advancing the case of the apparently unchanged mummies of Egyptian 

 animals against Lamarck's doctrine of descent, failed possibly through 

 the imperfectly discussed stage in which the whole question rested in 

 his day to understand that the very facts of preservation revealed in 

 the monuments of Egypt testified to the absence of those physical 

 changes which could alone have affected the animals of the Nile land. 

 But the fuller consideration of that theory of nature which credits pro- 

 gressive change as the usual way of life, shows us that it is no part of 

 evolution to maintain either that living beings must needs undergo 

 continual change, or that they must change and modify at the same 

 rate. On the contrary, Mr. Darwin, in his classic work, maintains ex- 

 actly the opposite proposition. There are, in fact, two great factors 

 at work in living nature a tendency to vary and change, and the in- 

 fluence of environments or surroundings. Given the first tendency, 

 which is not at all a matter of dispute, the influence of the second is 

 plainly enough discernible in bringing to the front either the original. 



Figs. 6 axd 7. Ichthyosaurus and Plesiosaurus. 



primitive, or, as it might be named, the parent form, or the varying 

 forms which are produced by modification of the parent. As it has 

 well been put : " Granting the existence of the tendency to the pro- 

 duction of variations, then, whether the variations which are produced 



