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 only 

 analogical when whales are compared with fishes, being 

 adaptations in both classes for swimming through the 

 water; but the shape of the body and fin-like limbs 

 serve as characters exhibiting true affinity between the 

 several members of the whale family ; for these ceta- 

 ceans agree in so 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 increasing 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 



