of these distinct forms (T. hecabe
and T. Aesiope), with several intermediates, from one batch of
caterpillars found feeding together on the same plant.[18] It is
therefore very probable that a considerable number of supposed distinct
species are only individual varieties.

Cases of variation similar to those now adduced among butterflies might
be increased indefinitely, but it is as well to note that such important
characters as the neuration of the wings, on which generic and family
distinctions are often established, are also subject to variation. The
Rev. R.P. Murray, in 1872, laid before the Entomological Society
examples of such variation in six species of butterflies, and other
cases have been since described. The larvae of butterflies and moths are
also very variable, and one observer recorded in the _Proceedings of the
Entomological Society for_ 1870 no less than sixteen varieties of the
caterpillar of the bedstraw hawk-moth (Deilephela galii).

_Variation among Lizards_.

Passing on from the lower animals to the vertebrata, we find more
abundant and more definite evidence as to the extent and amount of
individual variation. I will first give a case among the Reptilia from
some of Mr. Darwin's unpublished MSS., which have been kindly lent me by
Mr. Francis Darwin.

"M. Milne Edwards (_Annales des Sci. Nat._, I ser., tom. xvi. p. 50) has
given a curious table of measurements of fourteen specimens of Lacerta
muralis; and, taking the length of the head as a standard, he finds the
neck, trunk, tail, front and hind legs, colour, and femoral pores, all
varying wonderfully; and so it is more or less with other species. So
apparently trifling a character as the scales on the head affording
almost the only constant characters."

[Illustration: FIG. 1.--Variations of Lacerta muralis.]

[Illustration: FIG. 2.--Variation of Lizards.]

As the table of measurements above referred to would give no clear
conception of the nature and amount of the variation without a laborious
study and comparison of the figures, I have endeavoured to find a method
of presenting the facts to the eye, so that they may be easily grasped
and appreciated. In the diagram opposite, the comparative variations of
the different organs of this species are given by means of variously
bent lines. The head is represented by a straight line because it
presented (apparently) no variation. The body is next given, the
specimens being arranged in the