ORIGIN OF PLANT LIFE | 199 
inhabitants, and we must examine the relations exist- 
ing between the aquatic and the terrestrial species. 
As has been stated on a former page, the evidence 
points to life having originated in the water, at a 
period extremely remote. The most lowly as well as 
the most minute of all organisms are the bacteria, 
some of which are in size beyond the limit of the most 
powerful microscope to detect, their presence being 
known only by their chemical actions. The most 
primitive groups of bacteria, known as prototrophic, 
are able to live without light, deriving their nourish- 
ment by the breaking up of inorganic chemical 
compounds. It is difficult to conceive of any living 
organism more primitive than these, and quite possibly 
they recall that dim borderland where merely chemical 
structure and action mysteriously advanced into the 
cell structure and purposive chemical changes which 
we call life. From that lowly stage the evolution of 
plant life has been marked especially by three great 
forward bounds, of inestimable importance. The first 
of these was the “invention” of chlorophyll, which 
allowed plants to use for their life-processes the vast 
supply of energy furnished by the Sun. Sunlight then 
became essential to life, and the Algz, the probable 
ancestors of all the higher plants, were developed, 
presumably through the peculiar Cyanophycee, or 
“Blue-green Alge,” in which the chlorophyll is in a 
somewhat undifferentiated condition. Much later than 
this stage, yet far back in the history of evolution, 
occurred the second of the great forward steps. This 
was the desertion of the water for the land, which 
opened up for the plant world vast new fields and a 
great variety of new conditions. The final stage was 
