171 
they break up the carbon dioxide of the atmosphere by means 
of chlorophyll-bearing cells, and so on. 
Instead of vainly striving to cramp nature in the bonds of 
logic let us recognise this excessive elasticity of living forms. 
The late Professor Har vey has made such excellent ‘remarks 
on this subject in the introduction to.a book, now become 
rare, Manual of the British Marine Algae (1849. Van Voorst), 
that I shall take the liberty of quoting them :— 
«< Whoever has paid the slightest attention to the classifica- 
tion of natural objects, whether plants or animals, must be 
aware that, if we desire to follow-natural principles in forming 
our groups,—that is, to bring together such species as resemble 
each other in habits, properties, and structure,—it is a vain 
task to attempt to define, with absolute strictness, the classes 
into which we are forced to combine them. At least, no effort 
to effect this desirable object has yet been successful . 
But it fortunately happens that these difficulties are much 
more formidable on paper than in the field. 
he search into structure and affinities among the works of 
creation is something like that after first principles. We can 
distinguish and analyse up toacertain point; there we are 
stopped by that invisible and intangible, but impassable veil, 
behind which the Creator hides his operations. At this point 
we must rest satisfied with differences which we can see, but 
which we cannot know or define” (pp. ix. and x. of Intro- 
duction). 
The second great group of Cryptogams is the Moss alliance. 
Tiny as are most of its members, they generally possess a dis- 
tinct stem and leaves, and are invariably separated from Thallo- 
phytes by what is known as an alternation of generations, 
that is, by the occurrence of one form of the plant producing 
antheridia and archegonia, and of a second form arising as a 
peculiar result of the fertilised archegoniwm, the spore-capsule, 
familiar to us in Bryaceew as the elegant Urn-fruit. Morpho- 
logically, this fruit is, as it were, a graft on the mother plant, 
and constitutes a phenomenon so isolated as to give a high 
value in a systematic point of view to the Muscinee. Dr. 
Goebel, in a recent monograph on the mosses (Schenk’s 
Handbuch der Botanik, vol. ii. p. 401), says :—‘‘ We must 
accordingly be contented with affirming that the gulf between 
Mosses and Pteridophytes is the deepest that we know in the 
vegetable kingdom, and it is not made less by being bridged 
over by hypotheses and surmises.’ 
The third great group, the Pteridophytes or Fern type, 
is of immense importance from its prominence in geo- 
logical history. It is best divided into three classes, formed 
N 2 
