’ - ; “wie 
360 Geographical Distribution of Crustacea. 
may hence ke supposed to owe their distribution to independent 
creations of the same species in different places, as well as to mi- 
gration. Yot we may in this woderrate the exactness of physical 
identity required in regions for independent creations of the same 
spe 
cies. We know that for some chemical compounds, the condi- _ 
tion of physical forces for their férmation is exceedingly delicate ;_ 
and much more should we infer that when the creation of a living 
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, 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 
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 
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 t 
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 spe 
cies, we can point out none. Lach species of Crustacea may 
have had its place of origin and single centre of diffusion 
many and perhaps the majority of cases. But we have no ree 
i ee 
son to say that certain regions were without life, and were pe” 
