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 onlj-- 

 suggest the directions in which an exjjlanation 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 Em-asiatic 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 



