FACILITATES INTER-BEEEDING. 281 



districts proves at once that the constant currents acting in 

 these regions have prevented the separation of the varieties that 

 probably occur, into distinct species ; for we know from Dar- 

 win, that a new species can never be in a position to maintain 

 itself unmixed, if the individuals which have been modified by 

 some influence are not prevented from breeding in again with 

 the parent stock. But the currents which keep the Indian 

 Ocean and Red Sea in communication with the Pacific Ocean, 

 being constant, can just as well transport the parent forms as 

 the varieties, and consequently a free crossing between all the 

 forms derived from one stock is not only facilitated but rendered 



Fig. 72.— Onchidium tonganum, natural size. 



inevitable. The Crustacea and Fishes of the Red Sea correspond 

 very exactly with those of the Indian Ocean and Philippine Sea; 

 more than half the genera -and a large number of the species 

 are actually identical in these two very widely remote regions. 

 Even species which have accommodated themselves to a quite 

 peculiar mode of life are sometimes dispersed throughout these 

 regions; thus, for instance, the Cryptochirus of the Red Sea 

 appears to be in no respect specifically different from that of the 

 Philippines, and the Ha2mlocarcinus of the Pacific, described by 

 Stimpson, is found, it would seem identical in species, at the 

 Isle of Bourbon not far from Africa. Onchidium virrucula- 



