96 THE BEAUTIES OF NATURE chap. 



about, having evidently not the slightest idea 

 of their way home. They certainly did not 

 appear to possess any " sense of direction." 



NUMBER OF SPECIES 



The total number of species may probably 

 be safely estimated as at least 2,000,000, of 

 which but a fraction have yet been described 

 or named. Of extinct species the number 

 was probably at least as great. In the 

 geological history of the earth there have 

 been at least twelve periods, in each of which 

 by far the greatest number were distinct. The 

 Ancient Poets described certain gifted mortals 

 as having been privileged to descend into the 

 interior of the earth, and exercised their 

 imagination in recounting the wonders thus 

 revealed. As in other cases, however, the 

 realities of Science have proved far more 

 varied and surprising than the dreams of 

 fiction. Of these extinct species our knowl- 

 edge is even more incomplete than that of 

 the existing species. But even of our contem- 



