110 NATURAL SCIENCE [August 
of science. In the history of botany the same struggle about poly- 
morphism has taken place twice, first in the case of the fungi and 
then in that of the bacteria. On both occasions as it gradually be- 
came recognised that the pure culture of the organisms in question 
is the necessary starting point of every research, the fruitless 
theoretical discussions were replaced by work on really scientific 
lines. Likewise in the present case; for the solution of the 
problems with which they were concerned Chodat and Borzi 
ought to have started with pure cultures ; but this is just what 
they failed to do, The method of pure cultivation has hitherto played 
no part inalgology ; our knowledge has been obtained by the method 
of direct observation, which has been employed in a most thorough 
way by such investigators as Thuret, Cohn, Pringsheim, De Bary, 
and others. The fact is that the distinctive characters of many 
algae are quite sufficient to allow of their recognition among other 
organisms, and to enable their development to be followed by con- 
tinuous direct observation; and the artificial cultivation of many 
of these algae is beset with great difficulties. There are also 
many of the lower algae among which no- confusion arises in 
impure cultures. Forms such as Hydrodictyon, for instance, allow 
of their entire development being observed in the presence of other 
larger or smaller forms. But the matter is quite otherwise in all 
those algae which possess but few external readily recognisable 
characters, and among which at the same time there is such a 
multiplicity of closely allied species that all possible so-called 
transitional forms exist. This is the case, for instance, in the 
subaerial Ulothriz-like organisms, and also among the lower green 
algae such as the numerous species of Plewrococcus, Protococcus, 
Palmelia, etc., which are extremely easy to confound with one 
another. Furthermore, the higher algae possess developmental 
stages which to outward appearance are exactly like certain 
lower forms; and the false conclusion that therefore all lower algae 
are developmental forms of higher algae is often enough drawn. 
Swarming gametes of Chlamydomonas cannot be distinguished 
from gametes of Ulothrix; but it does not follow that Chlamy- 
domonas belongs to Ulothrix. Similarly filamentous algae form 
Protococcus-like stages ; but again it does not follow that all species 
of Protococcus belong to filamentous algae. Simple similarity in 
appearance or the apparent identity of two forms decides nothing, 
and anyone who relies upon it runs the risk of falling into the 
grossest error. Even two such distinguished investigators as Rosta- 
finski and Woronin were baffled in their observations on Botrydiwm 
simply because the developmental stages of the real Botrydivwm and 
of Protosiphon have a striking external resemblance. 
The question now presents itself, how must we proceed in order 
. 
