1590 CRUSTACEA. 



seas, when the conditions seem to be the same, favours the view that 

 migration has been the main source of the diffusion in the Oriental 

 tropics. 



As we descend in the order of Invertebrates, the species are less 

 detailed in structure, with fewer specific parts and greater simplicity 

 of functions, and they therefore admit of a wider range of physical 

 condition ; the same argument against multiplication by independent 

 creations in regions for the most part different, does not, therefore, so 

 strongly hold. As we pass, on the contrary, to the highest groups in 

 Zoology, the argument receives far greater weight; and at the same 

 time there are capabilities of migration increasing generally in direct 

 ratio as we ascend, which are calculated to promote the diffusion of 

 species, and remove the necessity of independent creations. 



Migration cannot therefore be set aside. It is an actual fact in 

 nature, interfering much with the simplicity which zoological life in 

 its diffusion would otherwise present to us. Where it ends, and 

 where independent creations have taken place, is the great problem 

 for our study. This question has its bearings on all departments of 

 Zoology ; but in few has migration had the same extended influence 

 as in that of Crustacea. Molluscs, if we except oceanic species, are 

 no travellers, and keep mostly to narrow limits. 



XIII. There is evidence in the exceedingly small number of Torrid 

 zone species identical in the Atlantic and Indian Oceans, that there 

 has been no water communication across from one to the other in the 

 Torrid zone, during the period since existing species of Crustacea 

 were first on the globe. 



XIV. As to zoological centres of diffusion for groups of species, we 

 can point out none. Each species of Crustacea may have had its 

 place of origin and single centre of diffusion in many and perhaps the 

 majority of cases. But we have no reason to say that certain regions 

 were without life, and were peopled by migration from specific centres 

 specially selected for this end. If such centres had an existence, 

 there is at present no means by which they may be ascertained. The 

 particular temperature region in which a species originated may be 

 ascertained by observing which is most favourable to its develop- 

 ment : we should thus conclude that the Ranina dentata, for example, 

 was created in the subtorrid region and not the torrid, as it attains its 

 largest size in the latter. By pursuing this course with reference to 

 each species, we may find some that are especially fitted for almost 



