388 ON THE ORIGIN OF SPECIES 



between the many living and extinct members of the 

 same great natural class. 



Extinction, as we have seen in the fourth chapter, 

 has played an important part in denning and widening 

 the intervals between the several groups in each class. 

 We may thus account even for the distinctness of 

 whole classes from each other — for instance, of birds 

 from all other vertebrate animals — by the belief that 

 many ancient forms of life have been utterly lost, 

 through which the early progenitors of birds were 

 formerly connected with the early progenitors of the 

 other vertebrate classes. There has been less entire 

 extinction of the forms of life which once connectee 

 fishes with batrachians. There has been still less ii 

 some other classes, as in that of the Crustacea, for 

 here the most wonderfully diverse forms are still tied 

 together by a long, but broken, chain of affinities. 

 Extinction has only separated groups : it has by nc 

 means made them ; for if every form which has ever 

 lived on this earth were suddenly to reappear, though 

 it would be quite impossible to give definitions by whicl 

 each group could be distinguished from other groups, 

 as all would blend together by steps as fine as those 

 between the finest existing varieties, nevertheless 

 natural classification, or at least a natural arrange 

 ment, would be possible. We shall see this by turning 

 to the diagram : the letters, A to L, may represent 

 eleven Silurian genera, some of which have producec 

 large groups of modified descendants. Every inter- 

 mediate link between these eleven genera and theii 

 primordial parent, and every intermediate link in each 

 branch and sub -branch of their descendants, may be 

 supposed to be still alive ; and the links to be as fine as 

 those between the finest varieties. In this case it would 

 be quite impossible to give any definition by which the 

 several members of the several groups could be dis- 

 tinguished from their more immediate parents ; or these 

 parents from their ancient and unknown progenitor. 

 Yet the natural arrangement in the diagram would still 

 hold good ; and, on the principle of inheritance , all the 



