68 THE DARWINIAN THEORY 



to the newer ones, and all forms are now included 

 as varieties of one species. Over 200 varieties 

 have been discovered in enormous numbers ; one 

 characteristic form being -found in each horizon. 

 The simpler unornamented shells are from the 

 lowest layers ; the most recent forms being identical 

 with a form now living only in North America and 

 the fresh-water lakes of China, which was for- 

 merly described as a distinct genus. This evidence 

 was found since the publication of the " Origin 

 of Species " in 1859, and renders the record less 

 incomplete. 



Now that men realise the value of- Palaeontology, 

 more attention has been directed to the subject ; for 

 in all cases positive palseontological evidence may be 

 implicitly trusted, although negative evidence is 

 worthless. 



The Extinction of Species. 



When a species or group has once disappeared 

 there is no reason to suppose that the same identical 

 form ever reappears — i.e., its existence, so long as it 

 lasts, is continuous. 



The influence of the size of animals, and its bearing 

 on Extinction of Species, is of the greatest possible 

 interest and importance. Many zoologists hold the 

 view, in support of which evidence is steadily in- 

 creasing, that the primitive or ancestral members of 

 each group were of small size. Thus, in the case of 

 birds, on the whole small birds show more primitive 

 conditions of structure than the larger members of 



