50 On the Geographical Distribution of Crustacea. 



diverse. Comparatively few genera of Insects have as wide a 

 range as those of Crustacea ; and species, with rare exceptions, 

 have very narrow limits. AVhere 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 

 physical conditions of distant provinces is far more common and 

 more exact than in the temperate zone ; and hence it would 

 seem that we cannot safely appeal to actual differences as an 

 argument against the creation of a species in more than one place 

 in the tropics. The species spread over the Oriental torrid zone 

 may hence be supposed to owe their distribution to independent 

 creations of the same species in different places, as well as to mi- 

 gration. Yet we may in this underrate the exactness of physical 

 identity required in regions for independent creations of the same 

 species. We know that for some chemical compounds, the condi- 

 tion 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 ma- 

 terial employed 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 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 multi- 

 plication 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 capa- 

 bilities 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 whei'e 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 



