356 AFFINITIES CONNECTING [CHAP. XIV. 



suppose that the bizcacha has retained, by inheritance, more of 

 the characters of its ancient progenitor than have other Rodents ; 

 and therefore it will not be specially related to any one existing 

 Marsupial, but indirectly to all or nearly all Marsupials, from 

 having partially retained the character of their common pro- 

 genitor, or of some early member of the group. On the other 

 hand, of all Marsupials, as Mr. Waterhouse has remarked, the 

 Phascolomys resembles most nearly, not any one species, but the 

 general order of Rodents. In this case, however, it may be 

 strongly suspected that the resemblance is only analogical, owing 

 to the Phascolomys having become adapted to habits like those 

 of a Rodent. The elder De Candolle has made nearly similar 

 observations on the general nature of the affinities of distinct 

 families of plants. 



On the principle of the multiplication and gradual divergence 

 in character of the species descended from a common progenitor, 

 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 common pro- 

 genitor of a whole family, 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 the 

 species ; and they 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 so without this aid, we can understand the extraordinary 

 difficulty which naturalists have experienced in describing, with- 

 out the aid of a diagram, the various affinities which they perceive 

 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 defining and widening the intervals between 

 the several groups in each class. We may thus account 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 and at that time less differentiated 

 vertebrate classes. There has been much less extinction of the 

 forms of life which once connected fishes with batrachians. There 

 has been still less within some whole classes, for instance the 

 Crustacea, for here the most wonderfully diverse forms are still 



