44 DARWINISM 



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 Homopterous 

 Fulgoridse. Mr. Westwood in his Modern Classification of 

 Insects states that " the species of Gerris, Hydrometra, and 

 Yelia 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. 



