178 FRESHWATER FAUNA 



forms will entail the exclusion of others dependent upon 

 them, which might not otherwise be disqualified. 



The first stage in our inquiry is now completed, and we 

 may next turn to the consideration of the means by which 

 the marine ancestors of our freshwater fauna acquired 

 their existing habitat. 



There are at least three conceivable ways in which this 

 may have been accomplished. Marine animals may (1) 

 wander directly into rivers from the sea ; slow-moving 

 animals may go far with sufficient time ; stationary animals 

 may be carried, like Birnham wood ; (2) they may acquire 

 a terrestrial or marsh-loving habitat, and subsequently 

 exchange this for a fluviatile or freshwater one ; (3) the 

 area they inhabit may be bodily transformed by some 

 movement of the earth's crust into a more or less closed 

 basin ; during the conversion of this into a lake Nature 

 will perform Beudant's experiment on a larger scale, and 

 under much more favourable conditions than he could 

 command. 



If, now, we inquire by which of these ways freshwater 

 faunas have originated, the answer will be, by all of 

 them. The mammals and birds, insects and spiders, 

 referred to in our list (p. 167) are modified land animals 

 they have entered by the second route ; the fish, or some 

 of them at least, as well as some of the Crustacea, have 

 come in by the first way, as direct immigrants ; while 

 many of the remainder have probably been captured by 

 the closure of land-locked seas. Before examining this 

 subject in detail let us turn our attention for a moment 

 to the question of " establishment." To secure its place 

 in running water the freshwater animal must lose its 

 habit of propagation by free swimming larvae, more 

 especially if it be a slow-moving or fixed species; and 

 this, as a rule, we shall find it has done ; most freshwater 

 animals of this kind, as well as others which are actively 



