GEOGRAPHICAL DISTRIBUTION OF CRUSTACEA. 289 



depend, in great part, upon the locomotive powers with which 

 they are endowed, either in their adult or their young states ; and 

 also (in regard, at least, to all but the freely-swimming marine 

 species) to the continuity of a line of coast, from one point to any- 

 other, along which their migrations may be effected. The 

 existence of constant or periodical currents, too, such as the 

 Gulf Stream of Mexico, will often affect the distribution of 

 species ; thus it is probably to this cause, that we are to attribute 

 the presence of some American Crustacea on the shores of the 

 Canary Islands. 



901. The following are the general principles arrived at by 

 M. Milne-Edwards in regard to the influence of Temperature on 

 the Geographical Distribution of Crustacea. 



I. The different forms and modes of organisation of these animals 

 manifest themselves more, in proportion as ice pass from the Polar 

 Seas towards the Equator. Thus, on the coasts of Norway, 

 where there is frequently a vast multiplication of individuals of 

 the same species, the number of species is very small ; but the 

 latter increase rapidly as we go southwards. Thus the number 

 of species of Crustacea of the first two Orders, known to exist on 

 the coast of Norway and the neighbouring seas, is only sixteen ; 

 but eighty-two are known to be inhabitants of the western shores 

 of Britain, France, Spain, and Portugal; one hundred and fourteen 

 are known in the Mediterranean Sea; and two hundred and two in 

 the Indian Ocean. A similar increase may be observed in fol- 

 lowing the coast of the New World, from Greenland to the 

 Caribbean Sea ; the number of species of Decapods in the former 

 region being only twelve, whilst in the latter it is seventy-one. 



II. The differences of form and organisation are not only more 

 numerous and more characteristic in the warm than in the cold 

 regions of the globe ; they are also more important. The number of 

 natural groups, which we find represented in the Polar and Tem- 

 perate Regions, is much smaller than that of which we find types 

 or examples in Tropical Seas. In fact, nearly all the principal 

 forms, which are met with in colder regions, also present them- 

 selves in warm ; but a very large proportion of the latter have 

 no representatives among the former. Thus, of the three primary 

 groups, into which the Class is at first subdivided, the Xypho- 



