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- 
