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. This excellent observer has shown
that in some districts of South America, where, for instance, an Ithomia
abounds in gaudy swarms, another butterfly, namely, a Leptalis, is often
found mingled in the same flock; and the latter so closely resembles the
Ithomia in every shade and stripe of colour, and even in the shape of its
wings, that Mr. Bates, with his eyes sharpened by collecting during eleven
years, was, though always on his guard, continually deceived.
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