366 THE BOOK OF THE ANIMAL KINGDOM 



plains, which were, probably, fens, and continental animals and 

 men could come and go as their requirements demanded. 



CONCLUDING REMARKS.— For a more complete account of the 

 animals of long ago the reader must be referred to volumes specially 

 devoted to the subject. But even from such a general review as 

 has here been made, some idea may be formed of the moving 

 panorama of life, and of the strange, and even romantic, drama 

 that has been enacted. There has been much shifting of scenery, 

 many entrances and exits have been effected; altogether it has been 

 "a strange coming and going of feet." Nothing but the Eternal 

 Law is fixed. All living creatures are subject to variations, no 

 physical form is settled for eternity. The posterity of an animal 

 may preserve the ancestral type for a million years, and exhibit 

 promise of generations upon generations to follow. But a new 

 factor introduced into the environment may bring about its extinc- 

 tion in less than a century. When we consider the arrival of such 

 creatures as the Sabre-Toothed Tiger we can easily understand the 

 extinction of multitudes of beasts which had few, if any, means of 

 protecting themselves against its sly, subtle, but ferocious and 

 destructive incursions. If we are asked why animals have become 

 extinct, the answer is that they have not been capable of adapting 

 themselves to changed circumstances. That creature survives that 

 can readily adapt itself to changes in temperature, change of food 

 and habitat, and defend itself against the attacks of invaders. The 

 advent of carnivorous mammals wrought mighty changes, and 

 determined numerous extinctions. In considering causes of extinc- 

 tion we have to remember that even the geography of the world 

 is not settled. The continents of to-day did not always have their 

 present configuration. The waters of the restless sea cover ancient 

 land surfaces, and old sea bottoms have been upheaved into con- 

 tinents and mountain chains. Geological changes have been 

 accompanied by changes in both flora and fauna. Earth move- 

 ments may affect sea-currents and modify the winds, and these in 

 turn may make great differences in climate. Although geological 

 developments are usually gradual, creatures may not necessarily 

 modify themselves to meet even a gradual change. Failure in such 

 modification leads to inevitable extinction. Perhaps man has done 

 more to bring about the rapid extinction of animals, and even 



