FRESH-WATER ALG. 253 
than between those of any other family of plants. 
Every traveller is struck with the wonderful same- 
ness of fresh-water productions animal and vege- 
table all over the world. De Candolle has 
remarked that in large groups of plants that 
have many terrestrial and only a few aquatic 
species, the latter have a far wider distribution 
than the former. Brazil, whose land flora and 
fauna are quite different from those of Britain, 
has yet many fresh-water insects, shells and 
plants precisely similar. Many fresh-water and 
marsh plants have an immense range over con- 
tinents, extending even to the remotest islands ; 
while water-beetles and fresh-water mollusca 
present the same types all over the globe. This 
similarity of type arises doubtless from similarity 
of conditions, and also from the temporary nature 
of collections of fresh water as compared with 
land and sea, not giving time sufficient for the 
production of varieties or of new species, which 
we have reason to believe is a very slow process, 
constantly exposing species of restricted range to 
destruction, and allowing only families of wide 
distribution to be preserved, As a rule, certain 
forms of fresh-water alge are to be found only in 
certain localities. The most conspicuous species 
in stagnant waters are Oscillatorias and Zygnemas. 
Other species, such as Conferva glomerata, and 
