SECOND OBSTACLE 173 



are capable of living in fresh water,* and consequently 

 it cannot be merely the difference in composition of the 

 media alone which is responsible for their absence from 

 our lakes and rivers. Whether the experiment equally 

 demonstrates a difference in susceptibility of different 

 species to change of medium, might be doubted, because it 

 is just possible- that some other unfavourable conditions 

 were necessarily associated with this ; but, on the other 

 hand, there is independent evidence to suggest that this is 

 the case. The Baltic, for instance, on account of its land- 

 locked position, is half lake, half sea, fresher on the 

 eastern side, salter on the western ; and it is just those 

 species which proved most indifferent to the composition 

 of the water in Beudant's experiment (Balanus, mussel, 

 and cockle) which are found to live on the east side, while 

 Tellina, which showed less power of accommodation, is 

 confined to the western side. 



Thus it appears that the absence of salt from our 

 rivers, while, no doubt, a hindrance to the migration of 

 marine animals, is by no means absolutely prohibitive. 



A second check, probably of equal importance, is to be 

 found in the greater severity of a fresh water climate, as 

 opposed to that of the sea ; and this must be especially 

 operative in temperate regions : nearer the tropics it may 

 be less effective, though droughts afford unfavourable con- 

 ditions which must not be overlooked. Von Martens, who 

 was the first to call attention to this factor, remarks : " The 

 great richness of the sea is explained not only by its 

 greater extent, but also by its more uniform temperature. 

 The fresh waters stand in the same relation to it as a con- 

 tinental to an insular climate : their alternation of tem- 

 perature is the principal hindrance to their becoming 

 populous; and this attains its maximum by freezing in 



* See Semper, " Animal Life," International Science Series, and 

 Vernon, " Variation of Plants and Animals," in the same series, p. 271. 



