DISTRIBUTIONAL DIFFICULTIES. 335 
At present, I confess that I do not see my way to a 
perfectly satisfactory explanation of the absence of cray- 
fishes in so many parts of the world in which they 
might, d@ priori, be expected to exist; and I can only 
suggest the directions in which an explanation may be 
sought. 
The first of these is the existence of physical obstacles 
to the spread of crayfishes, at the time at which the 
Potamobine and the Parastacine stocks respectively began 
to take possession of the rivers, some of which have 
now ceased to exist; and the second is the probability 
that, in many rivers which have been accessible to cray- 
fishes, the ground was already held by more powerful 
competitors. 
If the ancestors of the Potamobine crayfishes originated 
only among those primitive crayfishes which inhabited the 
seas north of the miocene continent, their present limita- 
tion to the south, in the old world, is as easily intelligible 
as is their extension southward, in the course of the river 
basins of Northern America as far as Guatemala, but 
no further. For the elevation of the Eurasiatic high- 
lands had commenced in the miocene epoch, while the 
isthmus of Panama was interrupted by the sea. 
With respect to the Southern hemisphere, the absence 
of crayfishes in Mauritius and in the islands of the Indian 
Ocean, though they occur in Madagascar, may be due 
to the fact that the former islands are of comparatively 
late volcanic origin; while Madagascar is the remnant of 
