GEOGRAPHICAL DISTRIBUTION OF CRUSTACEA. 1589 



most trivial kind, and effects only the least essential of the parts of a 

 species. 



The continental species of Crustacea from the interior of different con- 

 tinents, are not in any case known to be identical ; and it is well under- 

 stood 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 varieties of climate are greater; the 

 elevation above the sea may vary widely; and numberless are the 

 diversities of soil and its conditions, and the circumstances above and 

 within it. Hence as the creation of each species has reference most 

 intimately to each and all of these conditions, as well as to other pros- 

 pective ends, an identity between distant regions is seldom to be 

 found, and the characteristic groups of genera are very widely 

 diverse. Comparatively few genera of Insects have as wide a range 

 as those of Crustacea; and species with rare exceptions, have very 

 narrow limits. Where the range of a species in this class is great, 

 we should in general look to migration as the cause rather than 

 original creation; but the considerations bearing on both should be 

 attentively studied before either is admitted as the true explanation. 



Throughout the warmer tropical oceans, a resemblance in the phy- 

 sical conditions of distant provinces is far more common and more 

 exact than in the Temperate zone. And hence it would seem that 

 we could not safely appeal to actual differences as an argument 

 against the creation of a species in more than one place. The species 

 spread over the Oriental Torrid zone may hence be supposed to owe 

 their distribution to independent creations of the same species in diffe- 

 rent places, as well as to migration. Yet we may in this underrate 

 the exactness of physical identity required for independent creations 

 of the same species. We know that for some chemical compounds, 

 the condition of physical forces for their formation is exceedingly 

 delicate ; and much more should we infer that when the creation of a 

 living germ was concerned, a close exactness in the conditions would 

 be required in order that the creation should be repeated in another 

 place. Infinite power, it is true, may create in any place; but the 

 creation will have reference to the forces of matter, the material em- 

 ployed in the creation. The few species common to the Oriental and 

 Occidental torrid seas seem to be evidence on this point ; the fact that 

 the Oriental species have so rarely been repeated in the Occidental 



398 



