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 Harvey 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 Yoorst), 

 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 



The search into structure and affinities among the works of 

 creation is something like that after first principles. We can 

 distinguish and analyse up to a certain 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 archegoriium,t'he spore-capsule, 

 familiar to us in Bryaccce as the elegant Urn-frnit. 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 Muscinece. Dr. 

 G-oebel, in a recent monograph on the mosses (Schenk's 

 Handbuck 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 



