82 
W. B. Crow 
from an evolutionary point of view. Some knowledge of the phylo¬ 
genetic relationships of the various forms is necessary to a clear 
understanding of the origin of the group as a whole and hence of the 
Protista in general. In these pages the characters of certain of the 
unicellular forms, belonging to the family Chroococcaceae, are analysed, 
with a view to ascertaining their importance in the evolution of the 
group, and hence in helping to establish a satisfactory system of 
classification. Now the characters used by systematists to distinguish 
the various members of the Cyanophyceae, particularly the unicellular 
types, are perforce of the nature of very minute differences, since 
these organisms show little morphological variety. The true value of 
the characters employed can only be judged by a study of the vari¬ 
ation exhibited among individuals of the various species, more par¬ 
ticularly such as occur under natural conditions, and by an examina¬ 
tion of the possible underlying factors whether internal or external. 
Cell-form 
The family Chroococcaceae includes the simplest known repre¬ 
sentatives of the Cyanophyceae. In its most elementary state the 
organism of such a genus as Chroococcus or Glceocapsa consists of a 
single spherical cell, the form here owing its origin to purely physical 
factors. It is, in fact, the same as that of any inert amorphous 
fluid body placed in a medium with which it is immiscible. This is 
the dominant form of the cell in the Chroococcaceae, and it should 
be noted that not only is it to be considered as the primitive one, but 
that there would be a tendency for cells which have taken on other 
forms in response to stimuli, whether of external or internal origin, 
to revert to the original spherical state when the disturbing factors 
are removed. Spherical organisms are not uncommon, and their 
form by itself by no means proves a primitive position in the scale 
of evolution. One of the commonest modifications seen in the Chroo¬ 
coccaceae is found in the polyhedral form taken on by the cells of 
many species as a result of mutual pressure where the cells are asso¬ 
ciated in groups. In many species of Chroococcus the angular charac¬ 
ter is seen to be directly correlated with the degree of crowding of 
the cells, those in the less dense colonies or parts of colonies being 
perfectly spherical. From the point of view of evolution, then, such 
changes of cell-form as are met with in certain individuals of species 
of Chroococcus are of no great significance. The degree of crowding 
of the cells depends on the rate of cell-division and the rate of muci¬ 
lage secretion, two factors which, as will be shown later, are very 
