92 
Review. 
the Desmids proper is the production of two embryos from the 
zygote, while the Mesotaeniaceae have four and the Zygnemaceae 
one. 
In the filamentous forms Professor Oltmanns makes Debarya 
the most primitive type on account of its mode of conjugation, and 
from this derives Zygnema, Spirogyra and Sirogonium on the one 
side and on the other Zygogonium and Mougeotia. He deliberately 
neglects the form of the chromatophore (which Palla made the 
basis of a classification of the filamentous Conjugatae adopted 
in the “Revision”) and quite frankly refuses to discuss the 
subject. This is, we think to be regretted, since tbe relative 
importance of such characters among the various groups of Algae 
appears to be a most interesting and important topic. We cannot 
enter into it in detail here, but may say that while Professor 
Oltmanns’ derivation of the various modes of conjugation is both 
interesting and ingenious, on his own shewing these processes 
sometimes vary to such an extent within the limits of a single 
genus or even of a single species that the propriety of using them 
to the exclusion of characters which are singularly constant within 
the genera must be seriously called in question ; while the absence 
of any attempt to shew how one well-marked type of chromato¬ 
phore can arise from another, how for instance the Spirogyra- type 
can be derived from the Debarya- type, is a serious defect when 
probable lines of evolution are being sketched out. We cannot 
resist the conviction that it is the cell-characters which have been 
constant for the longest time while the details of the conjugation 
of gametes, even the details of the behaviour of protoplasm upon 
which Professor Oltmanns lays stress, are much more likely to be 
easily modified. We should all admit, of course, that the incipient 
differentiation of sex seen in Spirogyra is an advance on the per¬ 
fectly isogamous conjugation of Debarya or Mougeotia. The point 
is that more taxonomic weight should be attached to differences of 
cell-structure, because these appear to be more constant characters. 
Having devoted so much space to a consideration of some of 
the more fundamental topics connected particularly with the more 
primitive green forms, there is but little left in which to do justice 
to the rest of Professor Oltmanns’ work. 
We should much like, for instance, to discuss in detail his 
classification of the unicellular “ Isokontae.” This certainly shews 
a great advance on any previous classification put forward in a 
general work, though we are not in agreement with some of the 
details. However, as our author truly remarks, the making of a 
