44 
DARWINISM 
CHAP. 
subject to great variation, so that there is much uncertainty as 
to the number of species; and variations are especially frequent 
in the Planorbidse, which exhibit many eccentric deviations from 
the usual form of the species— deviations which must often 
affect the form of the living animal. In Mr. Ingersoll’s Report 
on the Recent Mollusca of Colorado many of these extra¬ 
ordinary variations are referred to, and it is stated that a shell 
(Helisonia trivolvis) abundant in some small ponds and lakes, 
had scarcely two specimens alike, and many of them closely 
resembled other and altogether distinct species. 1 
The Variability of Insects. 
Among Insects there is a large amount of variation, though 
very few entomologists devote themselves to its investigation. 
Our first examples will be taken from the late Mr. T. Vernon 
Wollaston’s book, On the Variation of Species, and they 
must be considered as indications of very widespread though 
little noticed phenomena. He speaks of the curious little 
carabideous beetles of the genus Notiophilus as being 
“ extremely unstable both in their sculpture and hue ; ” of 
the common Calathus mollis as having “ the hind wings at 
one time ample, at another rudimentary, and at a third nearly 
obsolete and of the same irregularity as to the wings being 
characteristic of many Orthoptera and of the Horaopterous 
Fulgoridse. Mr. Westwood in his Modern Classification of 
Insects states that “ the species of Gerris, Hydrometra, and 
Velia are mostly found perfectly apterous, though occasionally 
with full-sized wings.” 
It is, however, among the Lepidoptera (butterflies and 
moths) that the most numerous cases of variation have been 
observed, and every good collection of these insects affords 
striking examples. I will first adduce the testimony of Mr. 
Bates, who speaks of the butterflies of the Amazon valley 
exhibiting innumerable local varieties or races, while some 
species showed great individual variability. Of the beautiful 
Mechanitis Polymnia he says, that at Ega on the Upper 
Amazons, “ it varies not only in general colour and pattern, 
but also very considerably in the shape of the wings, 
especially in the male sex.” Again, at St. Paulo, Ithomia 
1 United States Geological Survey of the Territories, 1874, 
