CRUSTACEANS. 621 



cially MILNE EDWARDS, have made copious contributions to it, 

 which however are confined principally to the order of the decapods. 

 With the different peculiarities that have an influence on the distri- 

 bution of species we are not yet satisfactorily acquainted. The 

 difference of salt-particles in the different seas comes here beyond 

 doubt into consideration, arid not the mean temperature alone ; the 

 relative depth also, the nature of the coasts, &c. On the whole, 

 the general rule, that animal forms, the genera and species, are 

 more numerous in proportion as we advance from the poles to the 

 equator, has not such an unlimited prevalence for marine animals. 

 In size also and in brilliancy of colour the animals of the polar seas 

 often contest the palm with those of the tropics. Yet the short- tailed 

 ten-footed crustaceans, so rich in species in the seas of the warm 

 regions of our globe, are almost entirely wanting in the neighbour- 

 hood of the poles. On the other hand, the northern regions are 

 rich in forms of amphipods, which necessitate the adoption of 

 many distinct genera ; so that the colder seas may be regarded as 

 the proper home, the true father-land of these Crustacea 1 . 



1 H. KROEYEK Naturhisiorisk TidssJcrift, iv. 1842, pp. 141 166. On the geographic 

 distribution of Crustacea compare LATREILLE Mem. du Museum, in. 1817, p. 45 ; or 

 Memoires sur divers sujets de VHist. not. des Insecles, de Geographic ancienne, &c. 

 Paris, 1819, 8vo. p. 174; and especially MILNE EDWARDS, Ann. des Sc. natur. sec. 

 SeVie, Tom. x. pp. 129 174. 



