C. E. Beecher — Origin and Significance of Spines. 267 



is believed that the great amount of differentiation found in 

 some isolated stocks has been brought about. Primarily then, 

 a favorable condition for nutrition is assumed, which is fol- 

 lowed by excessive numerical multiplication ; while the natural 

 variations are augmented and governed by the action of repro- 

 ductive divergence for which such conditions are favorable. 

 Secondarily, these variations are subjected to the influences of 

 cannibalistic selection, defense, offense, sexual selection, and 

 mimicry. 



In illustration of the amount of differentiation attained by a 

 single stock under favorable conditions, the Amphipod Crusta- 

 ceans, Gammarus and Allorchestes, found in lakes Baikal and 

 Titicaca, respectively, may again be noticed. 



In respect to the number of species, Gammarus is very 

 sparsely distributed over the world, though in Lake Baikal 

 alone a hundred and seventeen species have been described by 

 Dybowsky. 17 In contrast to this, it may be mentioned that but 

 four fresh-water species have been discovered in the whole of 

 Norway. In Lake Baikal, all the depths explored (to 1373 

 meters) have furnished species. Those living near the surface 

 are vividly colored, yet apparently make no attempts at con- 

 cealment. Many of the species are also highly spinose, though 

 not sufficiently armed to be protected from the fish. As these 

 Crustaceans are voracious creatures, the spinose character has 

 probably been favored by the agency of cannibalistic selection. 

 The lake has a number of species of isli for which the 

 Gammaridse furnish excellent food, 

 species of seal, predaceous fish, as 

 well as the native fishermen keep the 

 fish below the danger point, thus 

 allowing the Gammaridse to become 

 very abundant. 



Similarly, in Lake Titicaca, there 

 is a wonderful specific development 

 of a kindred Crustacean, Allorchestes. 

 One of the most spinose species (A. 

 ar hiatus) is also the commonest, and 

 according to Faxon 19 occurs in count- 

 less numbers (figure 50). 



Packard 54 shows that, among cer- 

 tain moths, the caterpillars as soon as 

 they acquired arboreal habits met 

 with favorable conditions in respect 

 to food, temperature, etc., and that 

 as spines and tubercles arose by 

 normal variation, such features being- 

 found useful for protection, were 

 therefore preserved and augmented. 



but the presence of a 



Figure 50. Allorchestes ar- 

 matus. A spiny amphipod 

 from Lake Titicaca. Female ; 

 dorsal view. Natural size = 

 9 U1 '". (After Faxon.) 



