44 Geographical Distribution of Crustacea. 
should in general look to migration as the cause rather than orig- 
inal ereation ; but the considerations bearing on both should be 
attentively studied before either is admitted as the true expla- 
nation. 
''hroughout the warmer tropical oceans, a resemblance in the 
physical conditions of distant provinces is far more common and 
more exact than in the 'l'emperate zone. And hence it would 
seem that we could not safely appeal to actual differences as an ar- 
geument against the creation of a species in more than one place in 
the tropies. 'lhe species spread over the Oriental ''ornd 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. 'T'he few species common to 
the Oriental and Occidental torrid seas seem to be evidence on 
this point. 'T'he fact that the Oriental species have so rarely been 
repeated in the Occidental seas, when the conditions seem to be 
the same, favors 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- 
plieation. 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 re- 
ceives far greater weight; and at the same time there are capa- 
bilities of migration increasing generally in direct ratio as we as- 
cend, 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.  Molluses, if 
we except oceanic species, are no travellers, and keep mostly to 
narrow limits. 
X Cu an me a e pas ipe 
