Geographical Distribution of Crustacea. 49 



be with reference to the wide distribution of species in the 

 Oriental tropics, as well as in the European temperate regions, 

 and the temperate zone of the South Pacific and Indian 

 Oceans. 



XII. With respect to the creation of identical species in di- 

 stant regions, we would again point to its direct dependence on 

 a- near identity of physical condition. Although we cannot admit 

 that circumstances or physical forces have ever created a species 

 (as like can only beget like, and physical force must result sim- 

 ply in physical force), and while we see in all nature the free act 

 of the Divine Being, we may still believe the connexion between 

 the calling into existence of a species and the physical circum- 

 stances surrounding it, to be as intimate nearly as cause and effect. 

 The Creator has, in infinite skill, adapted each species to its place, 

 and the whole into a system of admirable harmony and perfection. 

 In His wisdom, any difference of physical condition and kind of 

 food at hand, is sufficient to require some modification of the 

 intimate structure of species, and this difference is expressed in 

 the form of the body or members, so as to produce an exactness 

 of adaptation, which we are far from fully perceiving or compre- 

 hending with our present knowledge of the relations of species 

 to their habitats. 



When therefore we find the same species in regions of unlike 

 physical character, as, for example, in the seas of the Canaries 

 and Great Britain — regions physically so unlike — we have strong 

 reason for attributing the diffusion of the species to migration. 

 The difference between the Mediterranean and Great Britain 

 may require the same conclusion for the species common to these 

 seas. They are so far different, that we doubt whether species 

 created independently in the two could have been identical, of 

 even have had that resemblance that exists between varieties ; 

 for this resemblance is usually of the most trivial kind, and affects 

 only the least essential of the parts of a species. 



The continental species of Crustacea from the interior of dif- 

 ferent continents are not in any case known to be identical ; and 

 it is well understood that the zoological provinces and districts of 

 the land are of far more limited extent than those of the ocean. 

 The physical differences of the former are far more striking than 

 those of the latter. As we have observed elsewhere, the varie- 

 ties of climate are greater ; the elevation above the sea may vary 

 widely ; and numberless are the diversities of soil and its condi- 

 tions, and the circumstances above and within it. Hence, as the 

 creation of each species has had reference most intimately to each 

 and all of these conditions, as well as to other prospective ends, 

 an identity between distant continental regions is seldom to be 

 found, and the characteristic groups of genera are very widely 



Arm. ^ Mag. N. Hist. Ser. 3. Vol. xvii. 4 



