The geographical distribution of animals, with a study of the relations ...

30 DISPERSAL AND MIGRATION. [part i.act very powerfully in modifying the distribution of fresh-waterfish.Sea fish would seem at first sight to have almost unlimitedmeans of dispersal, but this is far from being the case. Temperatureforms a complete barrier to a large number of species, coldwater being essential to many, while others can only dwell inthe warmth of the tropics. Deep water is another barrier tolarge numbers of species which are adapted to shores andshallows ; and thus the Atlantic is quite as impassable a gulfto most fishes as it is to birds. MaDy sea fishes migrate to alimited extent for the purpose of depositing their spawn infavourable situations. The herring, an inhabitant of the deepsea, comes in shoals to our coast in the breeding season ;whilethe salmon quits the northern seas and enters our rivers, mountingupwards to the clear cold water near their sources to depositits eggs. Keeping in mind the essential fact that changes oftemperature and of depth are the main barriers to the dispersalof fish, we shall find little difficulty in tracing the causes thathave determined their distribution.Means of Dispersal of Mollusca.—The marine, fresh-water, andland mollusca are three groups whose powers of dispersal andconsequent distribution are very different, and must be separatelyconsidered. The Ptcropoda, the lanthina, and other groups offloating molluscs, drift about in mid-ocean, and their dispersalis probably limited chiefly by temperature, but perhaps also bythe presence of enemies or the scarcity of proper food. Theunivalve and bivalve mollusca, of which the whelk and thecockle may be taken as types, move so slowly in their adultstate, that we should expect them to have an exceedingly limiteddistribution; but the young of all these are free swimmingembryos, and they thus have a powerful means of dispersal, andare carried by tidesand currents so as ultimately to spread overevery shore and shoal that offers conditions favourable for theirdevelopment. The fresh water molluscs, which one might atfirstsuppose could not range beyond their own river-basin, areyet very widely distributed in common with almost all otherfresh water productions ; and Mr. Darwin has shown that this is