122 THE DARWINIAN HYPOTHESIS 



tlio opposite sides of the narrow isthmus of Panama. 

 \Vhcrever we look, then, living nature oilers 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. \Vhatever 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 of whose lapse they 

 are the imperfect, but the only accessible witnesses. Now, 

 throughout the greater part of this long series of stratified 

 rocks are scattered, sometimes very abundantly, multi- 

 tudes of organic remains, the fossilized exuviae 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 that breathe the 

 upper ah*. But, singularly enough, the majority of these 

 entombed species are wholly distinct from those that now 

 live. Nor is this unlikeness without its rule and order. 

 As a broad fact, the further we go back in time the less 

 the buried species are like existing forms ; and the further 

 apart the sets of extinct creatures are the less they are 

 like one another. In other words, there has been a regular 

 succession of living beings, each younger set being in a very 

 broad and general sense somewhat more like those which 

 now live. 



It was once supposed that this succession had been the 

 result of vast successive catastrophes, destructions, and 

 re-creations en masse. ; but catastrophes are now almost 

 eliminated from geological, or at least paleontological 

 speculation ; and it is admitted on all hands that the 

 seeming breaks in the chain of being are not absolute, but 

 only relative to our imperfect knowledge ; that species 

 have replaced species, not in assemblages, but one by one ; 

 and that, if it were possible to have all the phenomena of 

 the past presented to us, the convenient epochs and forma- 



