282 GEOGRAPHICAL DISTRIBUTION OF CRUSTACEA. 



again, roam freely through the open sea, and finally, that others 

 are only to be met with on the dry land, at a considerable dis- 

 tance from the shore. Thus we see that each species has an 

 appropriate kind of residence, for which it is peculiarly adapted 

 by its organisation and habits ; but it may be further stated, that 

 each species has an appropriate place of residence, which is very 

 much determined by the temperature of the region. It is true 

 that there are many species very extensively distributed ; but 

 this results from their being adapted, by some peculiarities of 

 structure and habit, which we cannot detect, to sustain life 

 under a considerable variety of external conditions ( 13). The 

 extent to which even these species are distributed, however, will 

 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. 



819. 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 we 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- 

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



