can perhaps understand how it is that a numerical parallelism has sometimes been observed between the 
 subgroups | subgroups 1869 |  | sub-groups 1866 |  
  
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 
 their 
valuation 
 is as yet | is as yet 1869 |  
| has hitherto been 1866 |  
  
arbitrary), could easily extend the parallelism over a wide range; and thus the septenary, quinary, quaternary, and ternary classifications have probably arisen. ↑| 2 blocks not present in  1866 1869 1872; present in  1859 1860 1861 |  |  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 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. 
 |  
  
↑| 3 blocks not present in  1859 1860 1861 1866 1869; present in  1872 |  |  It should be observed that the process of imitation probably never commenced between forms widely dissimilar in colour. 
But starting with species already somewhat like each other, the closest resemblance, if beneficial, could readily be gained by the above means; and if the imitated form was subsequently and gradually modified through any agency, the imitating form would be led along the same track, and thus be altered to almost any extent, so that it might ultimately assume an appearance or colouring wholly unlike that of the other members of the family to which it belonged. 
There is, however, some difficulty on this head, for it is necessary to suppose in some cases that ancient members belonging to several distinct groups, before they had diverged to their present extent, accidentally resembled a member of another and protected group in a sufficient degree to afford some slight protection, this having given the basis for the subsequent acquisition of the most perfect resemblance. 
 |  
    | 
 As the modified descendants of dominant 
 species, | species, 1859 1860 1866 1869 1872 |  | species 1861 |  
  
belonging to the larger genera, tend to inherit the 
 advantages | advantages 1861 1866 1869 1872 |  | advantages, 1859 1860 |  
  
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 
 within each class thus | within each class thus 1861 1866 1869 1872 |  
| thus 1859 1860 |  
  
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 orders, 
 and under | and under 1869 1872 |  | under 1859 1860 1861 1866 |  
  
still fewer 
 classes. | classes. 1869 1872 |  | classes, 1859 1860 1861 1866 |  
  
 ..| ..... 1869 1872 |  | and 1859 1860 1861 1866 |  
  
 ..| ..... 1869 1872 |  | all 1859 1860 1861 1866 |  
  
 ..| ..... 1869 1872 |  | in 1859 1860 1861 1866 |  
  
 ..| ..... 1869 1872 |  | one 1859 1860 1861 1866 |  
  
 ..| ..... 1869 1872 |  | great 1859 1860 1861 1866 |  
  
 ..| ..... 1869 1872 |  | natural 1859 1860 1861 1866 |  
  
 ..| ..... 1869 1872 |  | system. 1859 1860 1861 1866 |  
   As showing how few the higher groups are in number, and how widely 
 they are spread | they are spread 1869 1872 |  
| spread they are 1859 1860 1861 1866 |  
  
throughout the world, the fact is 
 striking, | striking, 1859 1860 1861 1866 1869 |  | striking 1872 |  
  
that the discovery of Australia has not added 
 an | an 1866 1869 1872 |  | a single 1859 1860 1861 |  
  
insect belonging to a new 
 class; | class; 1860 1861 1866 1869 1872 |  | order; 1859 |  
  
and that in the vegetable kingdom, as I learn from Dr. Hooker, it has added only two or three 
 families | families 1861 1866 1869 1872 |  | orders 1859 1860 |  
  
of small size.  |