Chap. XIII. CLASSIFICATION. 431 



a common parent, together with their retention by 

 inheritance of some characters in common, we can 

 understand the excessively complex and radiating 

 affinities by which all the members of the same family 

 or higher group are connected together. For the com- 

 mon parent of a whole family of species, now broken 

 up by extinction into distinct groups and sub-groups, will 

 have transmitted some of its characters, modified in 

 various ways and degrees, to all ; and the several 

 species will consequently be related to each other by 

 circuitous lines of affinity of various lengths (as may be 

 seen in the diagram so often referred to), mounting up 

 through many predecessors. As it is difficult to show 

 the blood-relationship between the numerous kindred 

 of any ancient and noble family, even by the aid of a 

 genealogical tree, and almost impossible to do this 

 without this aid, we can understand the extraordinary 

 difficulty which naturalists have experienced in describ- 

 ing, without the aid of a diagram, the various affinities 

 which they perceive between the many living and ex- 

 tinct members of the same great natural class. 



Extinction, as we have seen in the fourth chapter, 

 has played an important part in defining 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 connected 

 fishes with batrachians. There has been still less in 

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

 here the most wonderfully diverse forms are still tied 



