CLASSIFICATION 385 



many characters, great and small, that we cannot 

 doubt that they have inherited their 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 can per- 

 haps 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 sure to spread widely, and to seize on more and 

 more places in the economy of nature. The larger 

 and more dominant groups thus tend to go on increas- 

 ing in size ; and they consequently supplant many 

 smaller and feebler groups. Thus we can account for 

 the fact that all organisms, recent and extinct, are 

 included under a few great orders, under still fewer 

 classes, and all in one great natural system. As show- 

 ing how few the higher groups are in number, and 

 how widely spread they are throughout the world, the 

 fact is striking, that the discovery of Australia has not 

 added a single insect belonging to a new class ; and that 

 in the vegetable kingdom, 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 

 to show, on the principle of each group having generally 

 diverged much in character during the long-continued 

 process of modification, how it is that the more ancient 



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