Chap. XIV. Affinities connecting Organic Beings. 379 



Mr. Waterhouse has remarked that, when a member belonging 

 to one group of animals exhibits an affinity to a quite distinct 

 group, this affinity in most cases is general and not special ; thus, 

 according to Mr. Waterhouse, of all Eodents, the bizcacha is most 

 nearly related to Marsupials ; but in the points in which it ap- 

 proaches this order, its relations are general, that is, not to any one 

 marsupial species more than to another. As these points of affinity 

 are believed to be real and not merely adaptive, they must be due 

 in accordance with our view to inheritance from a common progeni- 

 tor. Therefore we must suppose either that all Eodents, including 

 the bizcacha, branched off from some ancient Marsupial, which will 

 naturally have been more or less intermediate in character with 

 respect to all existing Marsupials ; or that both Rodents and Marsu- 

 pials branched off from a common progenitor, and that both groups 

 have since undergone much modification in divergent directions. 

 On either view we must suppose that the bizcacha has retained, 

 by inheritance, more of the characters of its ancient progenitor than 

 have other Eodents ; 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 progenitor, 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 Eodents. 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 

 Eodent. 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 progenitor 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 con- 

 sequently be related to each other by circuitous lines of affinity of 

 various lengths (as maybe 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 extra- 



