C. E. Beecher — Origin and Significance of Spines. 133 



and it is now common to find several of the most highly col- 

 ored varieties feeding together on the same leaf. The modern 

 importation of pigs, sheep, and mice on the islands has intro- 

 duced an enemy to the terrestrial species, the effects of which 

 are already being noticed. In specific differentiation and in 

 individual variation, both Hyatt and Verrill regard the extra- 

 ordinary development 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 

 Gammants in Lake Baikal, 17 and of Allorchestes in Lake Titi- 

 caca, 19 seem to furnish parallel examples. Allorchestes 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 19 has described seven distinct 

 species, constituting the entire Crustacean fauna with the 

 exception of a species of Oypris. Several species are " remark- 

 able among the Orchestidas for their abnormally developed 

 epimeral and tergal spines." These and the species of Gam- 

 marns from Lake Baikal will be referred to again later in this 

 paper. It is simply desired here to indicate that these varia- 

 tions in Achatinella and Allorchestes have arisen from a single 

 parent stock, within a small geographic province. The natural 

 interpretation seems to be (a) that the environment is favorable, 

 as evinced from the great number of individuals ; (b) that this 

 has favored and increased the growth force ; and (c) that, 

 finally, the law of multiplication of effects, reproductive 

 divergence," the survival of the unlike, and the conservative 

 forces of natural selection and heredity have directed the 

 growth force, and produced the specific differentiation which 

 is now found. 



A factor of Evolution, called " Reproductive Divergence " 

 by Yernon, 67 seems to be operative here, since it affords an 

 explanation for a means of differentiation in a single stock 

 under a common environment. As this factor has but recently 

 been discussed, it may well be defined at this time, so as to 

 enable a direct application to be made. Reproductive diver- 

 gence assumes that in many species there will be greater fertility 

 between individuals similar in color, form, or size, than between 

 individuals not agreeing in these respects, and that in subse- 

 quent generations, the divergence will become progressively 

 greater in respect to the characteristic in question, so that 

 finally the original stock will become separated into distinct 

 varieties, sub-species, or species. 



