ALGyE. 143 



probably owes its origin to the very remarkable phe- 

 nomenon of the colouring of its waters. 



2. That this phenomenon, observed for the first 

 time in 1823 by M. Ehrenberg, in the bay of Tor 

 only, then again seen twenty years later by M. 

 Dupont, but in truly gigantic dimensions, is owing 

 to the presence of a microscopic alga, sui generis, 

 floating at the surface of the sea, and even less 

 remarkable for its beautiful red colour than for its 

 prodigious fecundity. 



3. That the reddening of the waters of the Lake 

 of Morat by an Oscillaria, which M. de Candolle has 

 described, has the nearest relation to that of the 

 Arabian Gulf, although the two plants are generically 

 very distinct. 



4. That, as we may well suppose, according to the 

 accounts of navigators, who mention striking in- 

 stances of the red colouring of the sea, these curious 

 phenomena, though not observed till quite recently, 

 have nevertheless without doubt always existed. 



5. That this unusual colouring of seas is not ex- 

 clusively caused, as Peron and some others seem to 

 think, perhaps as being chiefly zoologists, by the 

 presence of mollusca and microscopic animalcules, 

 but that it is often also due to the reproduction, 

 perhaps periodical, and always very prolific, of some 

 inferior alga, and in particular of the species of the 

 singular genus Trichodesmium. 



6. That the phenomenon in question, although 

 generally confined between the tropics, is however 

 not limited to the Red Sea, nor indeed to the Gulf of 

 Oman ; but that, being much more general, it is found 



