ORIGIN AND SIGNIFICANCE OF SPINES 65 



alone a hundred and seventeen species have been described 

 by Dybowsky." In contrast to this, it may be mentioned 

 that but four freshwater species liave been discovered in the 

 whole of Norway. In Lake Baikal all the depths explored 

 (to 1,373 metres) have furnished species. Those living near 

 the surface are vividly colored, yet apparently make no 

 attempts at concealment. 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 fish for which the Gammaridaj furnish excellent food, but 

 the presence of a species of seal, predaceous fish, as well as 

 the native fishermen, keep the fish below the danger point, 

 thus allowing the Gammaridee to become very abundant. 



Similarly, in Lake Titicaca there so 



is a wonderful specific development 

 of a kindred Crustacean Allorches- 

 tes. One of the most spinose spe- 

 cies (A armatus) is also the com- 

 monest, and according to Faxon ^* 

 occurs in countless numbers (fig- 

 ure 50). 



Packard ^ shows that, among 

 certain moths, the caterpillars as 

 soon as they acquired arboreal hab- 

 its met with favorable conditions 

 in respect to food, temperature, 

 etc., and that as spines and tuber- -Fiqvre so.— AUorchestes ar- 

 cles arose by normal variation, such maius, a spiny amphipod from 



» , , ■ J. ^ £ T £ Lake Titicaca ; female ; dor- 



features, being found useful for ^^^^.^^ Natak size = 9 mm. 

 protection, were therefore preserved (After Faxon.) 

 and augmented. 



The differentiation of Achatinella has already been dis- 

 cussed (p. 36) as affording a striking instance of free varia- 

 tion among the Mollusca. The evolution of the Tertiary 

 species of Planorlis at Steinheim, as described by Hyatt, ^^ 



5 



