63 On the Occurrence of Marine Animal Forms in Fresh Water. 



{Blemus fulvescens^) , observed by Audouin, which remains con- 

 cealed under stones during the flood-tide, and lives in places 

 which are not left bare by every ebb ; the other so-called marine 

 Insects generally live only in brackish water, or roam about 

 upon its surface, like Halobates, which is analogous to our 

 Hydrometra-\. As regards the Arachnida, the answer to the 

 inquiry concerning marine forms, depends upon whether the 

 Pycnoi/oniddi be included with them ; the most recent and im- 

 portant authorities answer this in the affirmative. Amongst 

 the Myriapoda, the occurrence of Glomeris ovalis in the sea is 

 very problematical ; at any rate, it does not live in Oceana Eu- 

 ropcEo, as Linnseus stated, nor are we acquainted with any fresh- 

 water Myriapoda. 



For the classes, therefore, the number of the exclusive and 

 common ones would be nearly equivalent. Of the seven primary 

 types of the animal kingdom, ou the contrary, only one, that of 

 the Echinodermata, is exclusively marine ; the others are com- 

 mon to the sea and fresh water, and the majority (4) also to the 

 land : none of them are wanting in the sea. We may therefore 

 establish the general proposition, that from the agreement in 

 family of an animal of unknown origin, with another, of which 

 the origin is known, we may in most cases (in the Crustacea in 

 three-fourths, in the Mollusca in nearly nine-tenths) arrive at a 

 probable (inductive) conclusion with regard to the derivation of 

 the unknown form ; and that the same applies for a fraction of 

 the orders and classes, which often rises to the half of the pri- 

 mary type to which they belong, and for the EchinodermataJ 

 even to unity. 



On the other hand, descending in the systematic scale, only 

 an inconsiderable number of genera (in the modern sense := 

 groups of species) are common to both media, even in the Fishes 

 probably not more than 1 per cent. ; and with regard to species 

 the number falls to in the Mollusca and Crustacea, except 

 some cases which are still doubtful [Paludinella thermalis or 

 acuta, Gammarus locusta) ; amongst Fishes, not only is the oc- 

 currence of Gasterosteus trachurus in the North Sea asserted by 

 all the ichthyologists of that region, from Gronovius to Nilsson, 



[* This Insect forms the type of Leach's genus xiepus, of which a second 

 species, the Aepus Robinii, has lately heen discovered on the coasts both 

 of France and England. Besides these, a considerable number of Beetles, 

 piincipally belonging to the extensive group of the Brachelytra, so many 

 of which are singular in their habits, are found upon our shores in very 

 similar positions. — Transl.] 



[t Or rather, Gerris. — Transl.] 



[_X This can only hold, with regard to the Echinodermata, if we regard 

 them, with the author, as representing a distinct primary type of animal 

 structure; this, however, is by no means generally admitted, — Transl.] 



