2 8 DE GENERA TION. 



the existence at the present day oilow forms of animals 

 is proved by the fact that we find in very ancient strata 

 fossil remains of animals which differ, ever so little, 

 from particular animals existing at the present 

 day ; for instance, the Brachiopods (lamp-shells), 

 Lingula and Terebratula, the King-crabs, and the 

 Pearly Nautilus are found living at the present day, 

 and are also found with no appreciable difference 

 in very ancient strata of the earth's crust ; strata 

 deposited so long ago that most of the forms of life 

 at present inhabiting the earth's surface had not 

 then been brought into existence, whilst other most 

 strange and varied forms occupied their place, and 

 have now for long ages been extinct. 



Whilst we are thus justified by the direct testimony 

 of fossil remains in accounting for some living forms 

 on the hypothesis that their peculiar conditions of 

 life have been such as to maintain them for an im- 

 mense period of time in statu quo unchanged, we 

 have no reasoji for applying this hypothesis, and this 

 only, to the explanation of all the more imperfectly 

 organised forms of animal or plant-life. 



It is clearly enough possible for a set of forces 

 such as we sum up under the head " natural selec- 



