MIGBATION OF SPECIES. 149 



water, or viae versa, from actually taking place mucli more fre- 

 quently i 



I have already indicated that very often the strength of the 

 current in a river, or the surf at its mouth, its temperature or 

 the kind of food it affords, must cause quite as great hindrances 

 to the passage of a marine animal into the fresh water as the 

 necessity for subsequently living in water devoid of salt. Thus, 

 for instance, the remarkably tender bodies of the larvce of the 

 Echinodermata, Ascidise, sea-anemones, Hydroid polyps, and 

 others, are scarcely fitted to overcome such impediments ; so that, 

 evenlmder the assumption that they might be capable of living 

 in water without salt, their transfer into fresh water seems to be 

 almost impossible; and this is stUl more probably the case 

 when the fully grown creatures — stich as Ascidians, Corals, 

 Polyps, and others — do not move freely on the sea-bottom, but 

 are permanently attached to it. But if we now leave out of 

 the question the other influences which are often combined with 

 the variable ajnount of salt in the water, and which shall be 

 discussed in another place, we have in the first place to deter- 

 mine the optimum as well as the extreme proportion of salt in 

 the water which may be advantageous to different animals, so as 

 to be able to estimate how far variations in its saltness may have 

 a selective influence on those living in it or migrating into it. 

 Secondly, we must deal with the question whether and how 

 far an alteration in the salt contents of the water is capable 

 of directly modifying the morphological characters of a species. 

 But first of all we must ascertain the mode by which the 

 salt held in solution in the water penetrates to the interior of 

 the body, where alone it can produce any effect. 



Claude Bernard has proved that salt, when in solution in 

 water, can penetrate the body of an aniinal without the 

 creature's agency, merely by the endosmotic action of the skin. 

 If a frog is placed in a vessel in salt water, in such a posi- 

 tion that it cannot swallow any salt, it will nevertheless be 

 found that its body soon contains salt. If it absorbs more 

 than it can bear, it will die, and its death will ensue all the 

 sooner, the stronger the solution is in the first instance. In 

 order to determine what is the minimum percentage of salt 



