BIT BY BIT INVENTION : TEETH AND SCALES 305 



unlikeness to other groups partly to the extinction of 

 connecting forms which once existed, partly to gradual 

 divergence. Divergence among species is a form of 

 division of labour. One buttercup, for instance, becomes 

 adapted to life among long grass, another to dry stony 

 ground, a third to life in cornfields, which are regularly 

 reaped and sown again. The more closely they become 

 adapted each to its own sphere, the more will they diverge 

 from one another. 



We now regard the buttercups as plants which have 

 diverged in comparatively recent times from one common 

 ancestor, and this, if we could recover it, we should very 

 likely pronounce to be a buttercup too. The gaps which 

 separate the buttercups from one another, and the wider 

 gaps between the buttercups and the anemones, or between 

 the buttercups and the hellebores we attribute mainly to 

 the disappearance of connecting forms. The gap between 

 the buttercups and the cinquefoils we believe to be far 

 wider, and their common ancestor must date immeasure- 

 ably farther back than either the common ancestor of 

 all the buttercups, or the common ancestor of all the 

 cinquefoils. 



Darwin's theory of the Origin of Species shows that 

 the principle of a natural classification of plants or animals 

 is descent, near or remote, from a common ancestor. It 

 may restore our confidence in logical principles, or in 

 natural classifications, whichever was shaken, to remark 

 that the Darwinian explanation causes every natural 

 classification of plants and animals, like every logical 

 classification, to rest upon a single basis. 



LIII. BIT BY BIT INVENTION: TEETH 

 AND SCALES 



Whenever we are able to study closely the conversion of 

 structures to new uses, we find that the process is gradual. 

 This is in general true of human inventions too, though 



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