MIGRATION OF AQUATIC ANIMALS. 143 



good swimmers, and as were eurythermal, and that they 

 must not have been subjected to the iujurious influence of a 

 sudden change of food and sudden transfer to the salt elements 

 of sea-water. It is commonly said to give one special in- 

 stance that the rich variety of forms in the fauna of the Red 

 Sea and the Mediterranean is caused by their high degree of 

 saltness. The former is 4'31 per cent., and the second 3 - 79 per 

 cent., at the surface; but this leaves out of account the fact 

 that a merely superficial current pours in incessantly through 

 the narrow straits which, in each case, divide the sea from the 

 adjoining ocean, while a contrary current at the bottom of the 

 sea carries the waters of the inland sea back to the ocean. 

 Since, therefore, most swimming creatures, and particularly the 

 larvse of non-migratory animals, swim close to the surface, many 

 more creatures in both seas must be brought in than are 

 carried out, and thus the rich variety of forms in them may 

 certainly quite as likely be caused by the direction of these cur- 

 rents as by their greater saltness. 



In the total absence of all experiments directed to these 

 points, we may set aside such vague speculations and pass on 

 to the discussion of those facts which seem to prove that no 

 perfectly hard and fast line of demarcation exists between 

 fresh- water and marine animals, and that it is not absolutely 

 impossible to accustom them to live in the element to which 

 respectively they are strangers. The general importance of 

 this question requires that we should enter into particulars. 



A. Fresh-water animals that live in the sea. We are ac- 

 customed to give the name simply of fresh-water animals to 

 such groups, species, genera, families, or orders, as live exclu- 

 sively or almost exclusively in fresh water. It is evident that 

 if they were to migrate into the sea they would be exposed to a 

 certain effect from the salt, and it may even be supposed that 

 this effect might be injurious and strong enough to make it 

 quite impossible fora fresh-water animal that had migrated into 

 the sea to continue to live. However, there are a great number 

 of so-called fresh- water forms which do actually live in the sea, 

 some as visitors and some as constant inhabitants. It can 

 hardly be necessary here to remind the reader of the well-known 



