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 in algology ; 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 Ulothrix-like organisms, and also among the lower green 

 algae such as the numerous species of Pleurococcus, Protococcus, 

 Palmetto,, 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 Iiosta- 

 finski and Woronin were baffled in their observations on Botrydium 

 simply because the developmental stages of the real Botrydium and 

 of Protosiphon have a striking external resemblance. 



The question now presents itself, how must we proceed in order 



