264 THE PHILOSOPHY OF BIOLOGY 



many Reptiles, and the volant Mammals among the 

 Chordates : such are, of course, only instances of the 

 less successful lines of evolution in these phyla. The 

 dominant Arthropods and Vertebrates are the Crustacea, 

 the Hymenopterous Insects, the Teleost and Elasmo- 

 branch fishes, and the terrestrial Mammals. The earth 

 belongs to Man, to the social and solitary Ants, Wasps 

 and Bees, the marine Crustacea, the Teleost fishes, 

 the Trees, Grasses, and unicellular Diatoms and Peri- 

 dinians, and to the putrefactive and prototrophic 

 Bacteria. These are the organisms in which life has 

 attained its fullest manifestations, and has been most 

 successful in its mastery over inert matter. 



In what kinds of activity and morphology, then, 

 has the vital impetus found most complete expression ? 

 We see at once that in relation to energetic processes 

 life has followed two divergent lines animal and 

 vegetable. There is no absolute distinction between 

 the energy-transformations which proceed in the living 

 plant and animal we return to this point later on 

 but we may trace an unmistakable difference in 

 tendency, that is, in the direction taken by evolution. 

 This difference we have already considered in an earlier 

 chapter, but we may illustrate it by considering a 

 lifeless earth, and also one tenanted only by plants, 

 or animals, or by both. 



In a lifeless earth all energetic processes would tend 

 continually toward a condition of stability. The crust 

 of the earth, that is, the part known to us by direct 

 observation, is made up of rocks and the remains of 

 rocks ; materials consisting of compounds of oxygen, 

 silicon, iron, aluminium, sodium, potassium, calcium, 

 and so on. They are substances which would be stable 

 but for the eroding action of water, the gases of the 

 atmosphere, and volcanic activity. But as volcanic 



