CEUSTACEA OF NEW ZEALAND. 269 



(2) Persistence of the optic lobes and optic nerves, but total atrophy of the rods and 



cones, retina (pigment), and facets ; or 



(3) Total atrophy of the optic lobes, optic nerves, and all the optic elements. [See 



83, p. 118.] 



If we consider the other modifications of the body, legs, antennae, &c., which Packard 

 also accounts for as " evidently the result of loss of sight " [83, p. 120], Ave still find the 

 same capriciousness, and even in a more marked degree. Thus, taking our New Zealand 

 forms, we find that Cruregens fontanus and Culliopius subterraneus have developed 

 additional sensory setae beyond what are usiially to be found in their surface relatives, 

 while apparently Gammariis fragilis and Crangonyx compactus, and certainly the two 

 species of Phreatoicus, have not. Again, in the species of Phreatoicus, in Gammarus 

 fragilis, and to a less degree in Callioplm subterraneus and Cruregens fontanus, the 

 body, antennae, and appendages are slender and elongated, while there is no sign of 

 a similar modification in Crangonyx compactus, which has the body normally stout, the 

 antennae and legs of only moderate length, and the uropoda even somewhat short and 

 stumpy. 



Many similar examples could doubtless be adduced from a review of the underoround 

 fauna of other covmtries. Thus Boruta tenebrarum [124, pp. 677-G87] does not appear 

 to have the body particularly slender or the appendages elongated, while the species 

 of Niphargus usually do possess these peculiarities ; in Niphurgus the outer branch 

 of the third uropoda is greatly elongated, in Gammarus fragilis the peduncle and 

 both branches of the third uropoda are similarly elongated, while again in Cranqonyx 

 mucronatus, Forbes, the elongation takes place, not in the third uropoda at all, but in 

 the telson ! 



These examples, showing a development apparently capricious and varying in its 

 direction in animals all subjected to the same or similar environment, appear to jjoint rather 

 to the action of Natural Selection seizing here upon one variation useful to the animal and 

 there upon another, and fixing and maintaining these variations just as we find it doino- 

 in the more complicated phenomena of surface life. Packard refers to the cave 

 Crustaceans as living " in a sphere where there is little, if any, occasion for struffo-liuo- 

 for existence between these organisms " [83, p. 110]. 



But surely here, as elsewhere, the animals tend to increase in a geometrical ratio, and 

 since all cannot live, must necessarily struggle among themselves for food, which is as 

 Packard points out, very scanty. The Ccecidotea and Crangonyx of the North-American 

 caves are, Packard states, eaten by the blind crayfish, and must therefore " struggle," in 

 the sense in wliich the word is used by Darwin, with their destroyer, and in this struo-o'le 

 they appear to have developed those additional olfactory sette, &c., mentioned by 

 Packard, which enable them more readily to escape their enemy. If they had no 

 occasion for struggling for existence, w^hy should these additional sense-organs be 

 developed at all ? 



Packard does not appear to have considered the action of Natural Selection on the 

 individuals of the same species, an action which results in the perfecting and maintainiu"- 

 in a state of perfection any organ that is of importance to the animal. It is, however, 



35* 



