THE ARGUMENTS FROM CLASSIFICATION. 361 



suits a further reason for inferring that these have been 

 evolved. 



§ 125 There is yet another parallelism of like meaning. 

 We saw (§ 101) that the successively- subordinate classes, 

 orders, genera, and species, into which zoologists and botan- 

 ists segregate animals and plants, have not, in realily, 

 those definite values conventionally given to them. There are 

 well-marked species, and species so imperfectly defined that 

 certain systcmatists regard them as varieties. Between 

 genera, strong contrasts exist in many cases ; and in other 

 cases, contrasts so much less decided as to leave it doubtful 

 whether they constitute generic distinctions. So, too, is it 

 with orders and classes : in some of which there have been 

 introduced intermediate sub-divisions, having no equivalents 

 in others. Even of the sub-ldngdoms the same truth holds. 

 The contrast between the MoUuscoicla and the Mollusca, is far 

 less than that between the Mollusca and the Annulosa ; and 

 there are naturalists who think that the Fertebrata are so 

 much more widely separated from the other sub-kingdoms, 

 than these are from one another, that the Vertehrata should 

 have a classificatory value equal to that of all the other sub- 

 kingdoms taken together. 



Now just this same indefiniteness of value, or incomplete- 

 ness of equivalence, is observable in those simple and com- 

 pound and re-compound groups, which we see arising by 

 evolution. In every case, the endeavour to arrange the 

 divergent products of evolution, is met by a difficulty Hkc 

 that which would meet the endeavour to classify the 

 branches of a tree, into branches of the first, second, third, 

 fourth, &c., orders — the difficulty, namely, that branches of 

 intei-mediate degrees of composition exist. The illustration 

 furnished by languages will serve us once more. Some dia- 

 lects of English are but little contrasted ; others are strongly 

 contrasted. The alliances of the several Scandinavian tongues 

 with one another are diflTerent in degree. Dutch is much 

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