AXIMAl MIGBATI0X3. 41 



region of parched soils; and similar excursions, although in tliis 

 case governed by reversed thermonietric conditions, are pnictistd 

 by the onager or wild ass of Tartary. Even the reindeer is to an 

 extent a migrant, since in both Russia and Chinese Tartary it de- 

 scends far southward in advance of a rigouious winter, and, indeed, 

 frequently reaches a lower latitude than any part of England, al- 

 though in Scandinavia the animal is rarely seen south of the sixty- 

 fifth parallel. 



It is not alone among the higher animals that the migratory in- 

 stinct is developed. Turtles, during the ovipodting season, move 

 in considerable numbers from one p;»rt of the sea to another, and 

 they are stated to find their way annually to the Island of Ascen- 

 sion, which is distant upwards of eight hundred miles from the 

 nearest continental land-mass." Fishes migrate in immense num- 

 bers, but the periodical shifting of the abodes of these animals is 

 directly conne<"ted with the processes of reproduction. Certain 

 fishes. !»s the salmon, shad, and smelt, ascend the waters of fresh- 

 water streams for the purpose of depositing their eggs: others, 

 again, as the herring and mackerel, frequent in immense shoals, 

 during the breeding season, the neighbourhood of the coast-line. 

 The young eel follows the line of the river-courses in myriads, 

 ascending all the tributary streams, and frequently overcoming 

 apparently impassable water - falls by squirming over the moss- 

 covered ledges on either side. Among insects, the devastating 

 migrations of the locust are proverbial, and similar illustrations of 

 the wandering instinct could be cited from other members of the 

 same class of animals. A rem:u-kable example of migration has re- 

 cently been observed in the case of a species of grapsoid crab i^Se- 

 sarma >) off Cape San Antonio, the western extremity of the Mand 

 of Cuba. 



Barriers to Migration, and Facilities for Dispersion. — It 

 has already been remarked that the interpolation of extensive and 

 elevated mountain-chains and of large bodies of water, and also 

 sudden changes in the physical character of a country, are insur- 

 mountable obstacles in the way of the migration or dispersion of 

 certain classes of animals. The most serious of these obstacles, as 

 affecting the disjiersion of the Mammalia, is of course that of large 

 bodies of water. We are well aware that the most experienced 

 swimmer among this cUiss of ;uiimals can accomplish by the nata- 



