THE GENERAL CHARACTERISTICS OF SHORE ANIMALS. 15 



logical or structural characteristic can be. This is the 

 danger associated with a possible influx of fresh water into 

 the shore area. In most cases where the shore is fringed 

 by a long stretch of rocks, these rocks are interpenetrated 

 by fresh-water streams, and the animals in the neighbour- 

 hood of these streams are liable to be overwhelmed by 

 floods. On a larger scale, rocks in the vicinity of rivers 

 are similarly liable to the influx of large bodies of fresh 

 water. As is well known, many fish are not only indifferent 

 to the contact of fresh water, but at the breeding season 

 actually court it. Among those which can alternate from 

 fresh to salt water without danger are the salmon, eels, 

 sticklebacks, and others. Not a few fish, again, are ex- 

 tremely sensitive to the action of fresh water, which seems 

 to produce an almost instantaneous paralysis. Among the 

 lower animals a good many of the Crustacea and some 

 shellfish or Molluscs haunt estuaries or the neighbourhood 

 of streams, and are indifferent to the presence of a consider- 

 able amount of fresh water. In the vast majority of cases, 

 however, especially in the case of animals without shells, 

 fresh water acts as a powerful poison. This is especially 

 interesting, because we know that the salinity of sea water 

 varies greatly ; thus the Mediterranean is very dense, while 

 the Baltic contains a very much smaller portion of dissolved 

 salts, and yet some animals inhabit both areas. Experiment 

 shows that while an animal will not support direct trans- 

 ference from one of these media to the other, it can be 

 gradually educated to do this, if the changes are made 

 sufficiently slowly. Part of the interest of the shore area 

 is that it affords constantly varying conditions of life, the 

 variations under ordinary circumstances being small enough 

 to allow the animals time to adapt themselves to the new 

 conditions. It is because of these constant variations that 

 evolution has proceeded so rapidly in the area. 



One other general point must be considered, and that is 

 the way in which the animals of the shore area are dis- 

 tributed. In the preceding pages some attempt has been 

 made to indicate the vicissitudes of shore life, and to suggest 

 the great variety of conditions which may prevail there. 

 One consequence of this is that particular shore animals are 

 often very local in their distribution. Obviously an animal 



