, 
8 THE DARWINIAN HYPOTHESIS I 
isthmus of Panama.! Wherever we look, then, 
living nature offers us riddles of difficult solution, 
if we suppose that what we see is all that can 
be known of it. 
But our knowledge of life is not confined to the 
existing world. Whatever their minor differences, 
geologists are agreed as to the vast thickness of the 
accumulated strata which compose the visible part — 
of our earth, and the inconceivable immensity of 
the time the lapse of which they are the imperfect 
but the only accessible witnesses. Now, through- 
out the greater part of this long series of stratified 
rocks are scattered, sometimes very abundantly, 
multitudes of organic remains, the fossilised 
exuvie of animals and plants which lived and 
died while the mud of which the rocks are formed 
was yet soft ooze, and could receive and bury 
them. It would be a great error to suppose that 
these organic remains were fragmentary relics. 
Our museums exhibit fossil shells of immeasurable — 
antiquity, as perfect as the day they were formed ; — 
whole skeletons without a limb disturbed ; nay, 
the changed flesh, the developing embryos, and 
even the very footsteps of primeval organisms. 
Thus the naturalist finds in the bowels of the earth 
species as well defined as, and in-some groups 
of animals more numerous than, those which 
breathe the upper air. But, singularly enough, 
the majority of these entombed species are wholly 
1 [See page 60 Note. ] 
