As species belonging to 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 subgroups of distinct classes.

A naturalist, struck with a parallelism of this nature, by arbitrarily raising or sinking the value of the groups in several classes (and all our experience shows that their valuation is as yet arbitrary), could easily extend the parallelism over a wide range; and thus the septenary, quinary, quaternary and ternary classifications have probably arisen.

There is another and curious class of cases in which close external resemblance does not depend on adaptation to similar habits of life, but has been gained for the sake of protection.

I allude to the wonderful manner in which certain butterflies imitate, as first described by Mr.

Bates, other and quite distinct species.