114 NATURAL SCIENCE [August 
algae, the difficulty of arranging the species into genera and families 
is extraordinarily great, since there has been an enormous multi- 
plicity of species production. There exist, between the species 
typical of the various genera, numerous other forms, varieties, etc., 
each of which is a perfectly constant type, but which confuse the 
limits between genera and families. Very soon the dictum which 
I laid down in connexion with my systematic working up of the 
Flagellata in 1892, will apply equally well to the algae, namely, 
that the more we take into consideration the multitude of forms, 
the more difficult to construct and the more artificial our system 
becomes. The contradiction between the constancy of the single 
form, whether we call it species or variety, and the variability of all 
characters within the limits of an extended circle of forms, be it 
genus or family, has not yet been explained; Darwinian teaching 
has brought clearly to light the existence of this contradiction, but 
it has not yet discovered how to resolve it. 
GEORG KLEBS. 
