THE MEANING OF EVOLUTION 263 



or which we can still trace in the existing remnants of 

 groups which were formerly more abundant than they 

 are now. 



Only among the existing Bacteria, chlorophyllian 

 organisms, Arthropods, and Vertebrates has the vital 

 impetus found its most complete manifestation, and 

 we may even narrow down the main path that evolution 

 has taken to certain groups in each of these phyla. 

 Some of the Bacteria those which are exclusively 

 parasitic in the bodies of the warm-blooded animals- 

 have adopted a most specialised mode of life, and may 

 even be said to exist only with difficulty, since the 

 healthy animal is able to destroy them. Only those 

 Bacteria living in the open or upon the dead tissues of 

 plants and animals have attained to real dominance. 

 Some green plants, like the Ferns, are far less abundant 

 now than they were in the past ; while the Fungi and 

 some other saprophytic and parasitic plants have 

 specialised in much the same way as have the parasitic 

 worms, and are restricted in their distribution. Marine 

 Algae are confined to a relatively narrow selvedge of 

 sea round the land margin. The great trees, the 

 grasses, and the microscopic green plants such as the 

 Diatoms and Peridinians, represent the truly dominant 

 organisms in the vegetable kingdom. On the side of 

 the Arthropods and Vertebrates there have been many 

 unsuccessful lines of evolution : the Trilobites, for 

 instance, in the former group ; and the armoured 

 Ganoid fishes, the armed Reptiles, the volant Reptiles, 

 and the giant Saurians and Mammals among the Verte- 

 brates. Among the existing Arthropods and Verte- 

 brates there are some smaller groups which persist, so 

 to speak, only with difficulty. Such are the Spiders, 

 Mites, and Scorpions among the Arthropods ; and the 

 Tunicates, the Dipnoan fishes, the tailed Amphibians, 



