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Show 374 CLASSIFICATION. (CHAP. XIIL belonging to one group of animals ~xhibits an affi?-ity to a quite distinct group, this affinity 1n most cases 1s general and not special : thus, fl:CC~rding to Mr. Waterhouse, of all Rodents the bizcacha 1s most nearly related to Marsupials ; but in the points in which it approacl~es this order, its relations are general, and not to any o~e marsupial species 1nore than to anot~er. As th~ points of affinity of the bizcacha to Marsupials are beheved to be real and not merely adaptive, they are due on my theory to inheritance in common. Therefore we must suppose either that all Rodents, including the bizcacha, branched off from some very ancient Marsupial, which will have had a character in some degree intermediate with respect to all existing Marsupials ; or that both Rodents and Marsupials branched off from a coinmon progenitor, and that both groups have since undergone much modification in divergent directions. On either view we may suppose that the bizcacha has retained, by inheritance, more of the character 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 progenitor, or of an 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 resmnblance 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 osbservations on the general nature of the affinities of distinct orders of plants. On the principle of the multiplication and gradual divergence in character of the species descended from 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 common parent of a whole family of species, now broken up by extinction into dis· CnAP. XIII.] CLASSIFICATION. 375 tinct groups and sub-e;roups, will have transmitted s of its characters, modified in various ways and de orne to all ; and the seve:·al species ~ill consequently be r!l'a~~~ to each other by cucuitous hnes of affinity of variou lengths (as. n1ay be seen in the diagram so often referre~ t~), mounting up through man~ predecessors. As it is difficult ~o show the bloo~-relat1onship between the nuroero~ ls k1ndred of a~y ancient and noble fa1nily, even by the aid of a gonealogwal tree, and almost impossible to do t~is wit~ out this ai.d, we can . understand the extraordinar~ .dlfficu~ty whwh na:turahsts have experienced in describing, Without the aid of a diagram the various affinitie~ which they perceive between, the' many living and ext.Inc~ members of the same great natural class. Ext1nctl?n, as we have s~en in t~e fourth chapter, has played an Important part 1n 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 vertebr. ate animals-by the belief that many ancient forms of hfe .have bee~ utterly lost, through which the early progenitors ?f buds were formerly connected with the early progenitors of the other vertebrate classes. There has been less entire extinction of the forms of life which o~ce con~ected fishes with batrachians. There has been st1llless In some other olasses, as in that of the Crustacea for here the most wonderfully diverse forms are still tied t?ge~her by a long, but broken, chain of affinities. Extmctwn has only .separateft groups: it has by no means m~de them; for 1f every form 'vhich has ever lived on th1~ e~rth w~re suddenly to reappear, though it would be quite Impossible to give definitions by 'vhich each group could be distinguished from other groups as all would bl~n~ toget~er. by steps as fine as those· bet~een the finest ex1stmg varieties, nevertheless a natural classific.ation, or at least a n_atural arrangement, would be possible. We shall see th1s by turning to the diagram: the letters A to ~' n1ay represent eleven Silurian genera, some of ~hich ave ~reduced. laro-e groups of modified descendants. Every mtermed1ate Jink between these eleven genera and |