70 PLANT LIFE. 



connection with insect fertilization ; but the seeds produced 

 by these flowers are for the most part sterile, and the entire 

 structure may be looked upon as one that is practically 

 useless to the plant, but which it has not yet been able to 

 arrest, whereas the seeds produced by the cleistogamous 

 flowers are all fertile, and produce vigorous plants. As to 

 whether this reversion to the old method of self-fertilization 

 carries along with it the disadvantages of this method in 

 the older sense as compared with the advantages of cross- 

 fertilization, or whether the new or cleistogamous mode of 

 fertilization embodies some power of invigoration equal to 

 that effected by cross-fertilization is not determined. If 

 the last idea proves to be correct, then, however much we 

 may regret the idea of the eventual disappearance of the 

 brilliant colours of flowers, from the plant's point of view 

 it will be a great saving of energy, if a modified form of 

 the old method of self-fertilization confers equal benefits 

 to those imparted by the entomophilous mode of cross- 

 fertilization, by being enabled to dispense with the varied 

 structures and secretions at present necessary to secure the 

 visits of insects. 



T/ie relative advantages and disadvantages of plant life as 

 compared with animal life in the struggle for existence can be 

 best understood by following briefly the two principal lines 

 of departure from what may be termed a common or primi- 

 tive stock, the Protozoa. The lowest members of this 

 group, as previously stated, owing to their scanty differenti- 

 ation, lack those characteristics which are considered typical 

 of plants and animals respectively; they are all aquatic in 

 habit and require organic food. From the primitive con- 

 dition animal characteristics are slowly evolved in such 

 groups as the Infusoria, where a more or less rudimentary 



