196 FIRST FORMS OF VEGETATION. 
of development, until at the summit we behold 
those vast differences which distinguish an ele- 
phant from a palm-tree. 
In this class of plants, minute and obscure 
although they are, the infinite resources of Creative 
power are perhaps more overwhelmingly revealed 
to our perceptions, than in even the highest orders 
of the vegetable kingdom. The most unwearied 
research, continued for centuries, has not yet as- 
signed limits to that amazing variety which is their 
most remarkable feature, numbering as they do 
species that baffle classification, and within which 
a still more astounding variety of individual types 
are to be found. 
Every one is familiar with that green slimy 
matter, which during the spring and summer 
months creams over the surface of the stagnant 
pool, the half dried-up streamlet, or the wayside 
ditch ; but there are few who regard it otherwise 
than asa disagreeable scum or impurity, to which in 
Scotland the expressive name of s/aak has been 
applied. It is in reality, however, an aggregation 
of plants, perfect in all their parts, and furnished 
with peculiar organs of nutrition and reproduc- 
tion. Let us place a small portion of it on a con- 
cave glass, containing a drop or two of water suf- 
ficient to float it freely, and then place it under 
the microscope for examination, and what a 
