y to be the same,
are essentially different. The principle formerly alluded to under the
term of ANALOGICAL VARIATION has probably in these cases often come into
play; that is, the members of the same class, although only distantly
allied, have inherited so much in common in their constitution, that
they are apt to vary under similar exciting causes in a similar
manner; and this would obviously aid in the acquirement through natural
selection of parts or organs, strikingly like each other, independently
of their direct inheritance from a common progenitor.

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. When the mockers and the mocked are caught and
compared, they are found to be very different in essential structure,
and to belong not only to distinct genera, but often to distinct
families. Had this mimicry occurred in only one or two instances, it
might have been passed over as a strange coincidence. But, if we proceed
from a district where one Leptalis imitates an Ithomia, another mocking
and mocke