306 THE PHILOSOPHY OF BIOLOGY 



movements of its roots, leaves, and stems. Instinctive 

 and intelligent behaviour coexist in many animals, 

 yet the tendency of man, most highly intelligent of 

 all, is more and more to act intellectually ; while the 

 opposing tendency, that is, to act instinctively, has been 

 evolved in the Hymenoptera. It seems as if such con- 

 trasting methods of transforming energy, or of acting, 

 were incompatible with each other, and yet it is clear 

 that they are not really incompatible, for they may 

 co-exist. But it does seem clear that each of these 

 contrasting tendencies cannot be manifested to the 

 fullest extent if it is accompanied by the other. That 

 is to say, life is limited in its power over inert matter. 

 Manifested in the same material constellation, it 

 cannot both use solar radiation to build up substances 

 of high potential energy and then break down these 

 substances so as to obtain kinetic energy of movement. 

 Now we see clearly that life on our earth is indeed 

 limited to a very restricted range of physical conditions. 

 When we think of the mass of the earth we are surprised 

 to find what an insignificant fraction of all this matter 

 displays vital phenomena. The surface of the land 

 is clothed with a layer of vegetation, luxuriant and 

 abundant as we see it when we walk through a tropical 

 forest, but which is really a film of inconceivable tenuity 

 when we compare its thickness with the diameter of 

 the globe. Even the whole surface of the land is not 

 so clothed with vegetation, for polar regions and the 

 tops of high mountains are almost lifeless, while desert 

 tracts may be absolutely so. The lower strata of the 

 atmosphere are inhabited by birds, insects, and bac- 

 teria, but the total mass of these is infinitesimal when 

 compared with the total mass of the gases of which the 

 atmosphere is composed. Even the sea, which we regard 

 as rich in life, is not really so : estimates of the luxuri- 



