18 Darwinism and Other Essays. 



quite competent to work deep-seated changes of 

 structure and function. 



When we consider, then, that the circumstances 

 which determine what individuals shall survive 

 are not constant in the long run for any species, 

 though apparently constant for limited periods of 

 time ; when we reflect that there is no one of 

 the larger groups of plants and animals such as 

 orders, or families, or even genera which have 

 not been subjected again and again to great and 

 complicated changes of environment, it becomes 

 evident that anything like " fixity of species " is 

 utterly out of the question. No such thing is 

 possible or even imaginable, when once the facts 

 of the case have been thoroughly conceived. 

 Looking over the earth's surface to-day, things 

 may seem quiet and stable enough. But if we 

 contemplate the succession of past events, as dis- 

 closed by the geologist, what mainly strikes our 

 attention is the secular turmoil. Islands aggre- 

 gating into continents ; continents breaking up 

 into archipelagoes ; rivers shifting their beds ; 

 coast-lines changing their direction ; oceans now 

 separated by impassable isthmus-walls, now min- 

 gling their floras and faunas through new-made 

 channels ; torrid zones becoming temperate, and 

 temperate zones growing frigid; marshes trans- 



