10 EVOLUTION OF PLANTS 
and structure, and constitute tissues of different kinds 
which are in turn aggregated to form special organs. 
Thus a special subterranean root system is present, and 
the green assimilative tissue is mainly confined to the 
leaves, which are preéminently organs for carbon assimi- 
lation. The extreme of specialization is reached in the 
flowers of these plants, which are beyond doubt the 
most complicated structures which occur in the plant 
kingdom. 
Between the extremes found in the unicellular plants 
at the bottom of the series, and the complicated seed 
plants at the top, are numberless gradations of structure 
which throw much light upon how these advances in 
structure have been brought about. A similar progress 
from the simple to the complex is, of course, evident in 
the evolution of the animal kingdom ; but the animal 
type reaches a far greater dégree of complexity and 
specialization than is ever found even in the highest 
plants, which differ much less from the lower ones than 
is the case among animals. 
At the bottom of the scale the two kingdoms con- 
verge. There are many forms to be met with whose 
position is more or less doubtful, and in some cases it 
is practically impossible to determine to which great 
division they belong. We can only say that we have 
to do with organisms which are not yet sufficiently 
differentiated to determine whether the animal or 
vegetable characters predominate. It is the study of 
these primitive organisms, and the realization of the 
close similarity in the structure and functions of the 
animal and plant cell, which emphasize the intimate 
connection between the two kingdoms, and the impos- 
