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, 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 within each class 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 orders and under still fewer classes.