36 STUDIES IN EVOLUTION 



In illustration of these points, the Achatinellae of the Sand- 

 wich Islands afford a good example. The great number of 

 species on these islands has probably been evolved since 

 Tertiary times, and the process of specific delimitation is 

 apparently still going on, for species are now to be found 

 which did not exist fifty years ago (Verrill); also, a few 

 species formerly common are now obsolescent or extinct. 

 According to Hyatt, they all can be deducible from a single 

 species which has differentiated in time through divergence, 

 dispersion, and colonial isolation. In early times birds may 

 have fed upon them, but the complete or partial extinction 

 of the former by man has resulted in complete immunity for 

 the arboreal Achatinellse, and it is now common to find 

 several of the most highly colored varieties feeding together 

 on the same leaf. The modern importation of pigs, sheep, 

 and mice on the islands has introduced an enemy to the 

 terrestrial species, the effects of which are already being 

 noticed. In specific differentiation and in individual varia- 

 tion, both Hyatt and Verrill regard the extraordinary develop- 

 ment of this type as characteristic of free variation, under 

 favorable conditions, in a plastic stock which has not yet 

 reached its limits nor become fixed. 



Among the Crustacea, the remarkable evolution of the 

 genus Gammarus in Lake Baikal,^" and of AUorchestes in 

 Lake Titicaca,^^ seem to furnish parallel examples. AUor- 

 chestes ranges from Maine to Oregon and southward, through 

 the United States, Mexico, and South America, to the Straits 

 of Magellan. Before Lake Titicaca was explored, but one 

 or two authentic freshwater species were known from both 

 continents ; yet from this lake basin alone, Faxon ^^ has 

 described seven distinct species, constituting the entire 

 crustacean fauna with the exception of a species of Oi/pris. 

 Several species are "remarkable among the Orchestidse for 

 their abnormally developed epimeral and tergal sjDines." 

 These and the species of Gammarus from Lake Baikal will 

 be referred to again later in this paper. It is simply desired 

 here to indicate that these variations in Achatinella and 



