214 THE BOOK OF NATURE STUDY 



the Glacial Epoch for the occurrence of glaciers at that place, 

 and that the climate of succeeding ages should never have been 

 so much improved as to kill off the survivors of that Epoch. 



In a similar way the story of life must have been strongly 

 influenced by geographical changes taking place in geological 

 times. Evolution steadily proceeding must have been at times 

 steadfast and regular as a consequence of stability in geographical 

 conditions throughout long periods. At other times changes 

 must have been accelerated by rapidly changing environment, 

 migration must have been expedited, and the conditions of 

 existence rendered so stringent that only the types with the most 

 elastic organisations could have survived. 



The study of fossils in association with the position, structures, 

 and relationships of the rocks has already given, and will continue 

 to give, new data with reference to those factors in organisa- 

 tion which make for survival, improvement, or extinction. And 

 a complete knowledge of the organisms of the past and the 

 environment in which they lived has become essential to 

 students of the life of the present day. 



