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Show 428 CLASSIFICATION. CHAP. XIII. the same class or order are compared one with another : thus the shape of the body and fin-like. limbs are o~ly analogical when whales are compa~ed ":Ith fishes, bmng adaptations in both classes for swimming thi~ough. the water; but the shape of the body an~ fin-hke hmbs serve as characters exhibiting true affinity between the several members of the whale family; for these cetaceans agree in so many characters, gr~at a:1d smal~, that we cannot doubt that they have Inhented thmr general shape of body and structure of limbs from a common ancestor. So it is with fishes. As members of distinct classes have often been adapted by successive slight modifications to live under nearly similar circumstances,-to inhabit for instance the three elements of land, air, and water,-we c~n perhaps understand how it is that a numerical parallelism has sometimes been observed between the sub-groups in distinct classes. A naturalist, struck by a parallelism of this nature in any one class, by arbitrarily raising or sinking the value of the groups in other classes (and all our experience shows that this valuation has hitherto been arbitrary), could easily extend the parallelism over a wide range ; and thus the septenary, quinary, quaternary, and ternary classifications have probably arisen. As the modified descendants of dominant species, belonging to the larger genera, tend to inherit the advantages, which made the groups to which they belong large and their parents dominant, they are almost su~·e to spread widely, and to seize on more and more pla~es m the economy of nature. The larger and more dommant groups thus tend to go on increasing in size ; and they consequently supplant many smaller and feebler gr?ups. Thus we can account for the fact that all orgamsms, recent and extinct, are included under a few great CHAP. XIII. CLASSIFICATION. 429 orders, under still fewer ?lasses, and all in one great nat~ral system. As show~ng how few the higher groups are In number, and how.wide~y.spread they are throughout the .world, the fact IS st~Iking,. that the discovery of Austraha has not added ·a Single Insect belongin()' to a new order; and that in the vegetable kingdom, 0 as I learn from Dr. Hooker, it has added only two or three orders of small size. In the chapter . on. geological succession I attempted t~ show, on the ~nnciple of each group having generally d1verged much In character during the long-continued process of modification, how it is that the more ancient forms of life often present characters in some slight degree intermediate between existing groups. A few old an~ intern1ediate parent-forms having occasionally transmitted to the present day descendants but little modified, will give to us our so-called osculant or aberrant groups. The more aberrant any form is, the greater must be the number of connecting forms which on my theory have been exterminated and utterly lost. And we have some evidence of aberrant forms having suffered severely from extinction, for they are generail~ represented by extremely few species; and such spemes as do occur are generally very distinct from eac~ other, which again implies extinction. The genera Orn1thorhynchus and Lepidosiren, for example, would not have been less aberrant had each been represented by a dozen species instead of by a sino-le one· but such . h b ' r1c ness in species, as I find after some investigation does not commonly fall to the lot of aberrant gener: We can, I think, account for this fact only by looking at aberrant forms as failing groups conquered by more successful competitors, with a few members preserved by some unusual coincidence of favourable circumstances. Mr. Waterhouse has remarked that, when a member |
