30 DISPERSAL AND MIGRATION. [PART 1. 
act very powerfully in modifying the distribution of fresh-water 
fish. 
Sea fish would seem at first sight to have almost unlimited 
means of dispersal, but this is far from being the case. Tempera- 
ture forms a complete barrier to a large number of species, cold 
water being essential to many, while others can only dwell in 
the warmth of the tropics. Deep water is another barrier to 
large numbers of species which are adapted to shores and 
shallows; and thus the Atlantic is quite as impassable a gulf 
to most fishes as it is to birds. Many sea fishes migrate to a 
limited extent for the purpose of depositing their spawn in 
favourable situations. The herring, an inhabitant of the deep 
sea, comes in shoals to our coast in the breeding season; while 
the salmon quits the northern seas and enters our rivers, mount- 
ing upwards to the clear cold water near their sources to deposit 
its eggs. Keeping in mind the essential fact that changes of 
temperature and of depth are the main barriers to the dispersal 
of fish, we shall find little difficulty in tracing the causes that 
have determined their distribution. 
Means of Dispersal of Mollusca—tThe marine, fresh-water, and 
land mollusca are three groups whose powers of dispersal and 
consequent distribution are very different, and must be separately 
considered. The Pteropoda, the Janthina, and other groups of 
floating molluscs, drift about in mid-ocean, and their dispersal 
is probably limited chiefly by temperature, but perhaps also by 
the presence of enemies-or the scarcity of proper food. The 
univalve and bivalve mollusca, of which the whelk and the 
cockle may be taken as types, move so slowly in their adult 
state, that we should expect them to have an exceedingly limited 
distribution; but the young of all these are free swimming 
embryos, and they thus have a powerful means of dispersal, and 
are carried by tides and currents so as ultimately to spread over 
every shore and shoal that offers conditions favourable for their 
development. The fresh water molluscs, which one might at 
first suppose could not range beyond their own river-basin, are 
yet very widely distributed in common with almost all other 
fresh water productions ; and Mr. Darwin has shown that this is 
