The most important negative fact which remains to be accounted for is the absence of crayfishes in the rivers of a large moiety of the continental lands, and in numerous islands. Differences of climatal conditions are obviously inadequate to account for the absence of crayfishes in Jamaica, when they are present in Cuba; for their absence in Mozambique, and the islands of Johanna and Mauritius, when they are present in Madagascar; and for their absence in the Nile, when they exist in Guatemala. {335} [42] Just as there is an American form of _Idothea_ and an Asiatic form in the Arctic ocean at the present day. At present, I confess that I do not see my way to a perfectly satisfactory explanation of the absence of crayfishes in so many parts of the world in which they mighty _à 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 crayfishes, 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 limitation 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 highlands 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 {336} a very ancient continental area, the oldest indigenous population of which, in all probability, is directly descended from that which occupied it at the beginning of the tertiary epoch. If Parastacine _Crustacea_ inhabited the southern hemisphere at this period, and subsequently became extinct as marine animals, their preservation in the freshwaters of Australia, New Zealand, and the older portions of South America may be understood. The difficulty of the absence of crayfishes in South Africa[43] remains; and all that can be said is, that it is a difficulty of the same nature as that which confronts us when we compare the fauna of South Africa in general with that of Madagascar. The population of the latter region has a more ancient aspect than that of the former; and it may be that South Africa, in its present shape, is of very much later date than Madagascar.