102 LECTURES ON EVOLUTION m 



hitherto known, the tail is relatively short, and 

 the vertebrae which constitute its skeleton are 

 generally peculiarly modified. 



Like the Anoplotherium and the Palceotherium, 

 therefore, Archceopteryx tends to fill up the interval 

 between groups which, in the existing world, are 

 widely separated, and to destroy the value of the 

 definitions of zoological groups based upon our 

 knowledge of existing forms. And such cases as 

 these constitute evidence in favour of evolution, 

 in so far as they prove that, in former periods of 

 the world's history, there were animals which over- 

 stepped the bounds of existing groups, and tended 

 to merge them into larger assemblages. They 

 show that animal organisation is more flexible than 

 our knowledge of recent forms might have led 

 us to believe ; and that many structural permuta- 

 tions and combinations, of which the present 

 world gives us no indication, may nevertheless 

 have existed. 



But it by no means follows, because the Palceo- 

 therium has much in common with the horse, on 

 the one hand, and with the rhinoceros on the 

 other, that it is the intermediate form through 

 which rhinoceroses have passed to Income horses, 

 or vice versa ; on the contrary, any such supposition 

 would certainly be erroneous. Nor do I think it 

 likely that the transition from the reptile to the 

 bird has been effected by such a form as Archce- 

 opteryx. And it is convenient to distinguish these 

 intermediate forms between two groups, which do 



