THE MEANING OF EVOLUTION 259 



plants and insects must have been able to traverse 

 oceanic areas. The abundance of a group obviously 

 implies great powers of reproduction, the ability to 

 withstand physical changes, and the ability to resist 

 competition with other predatory creatures. Domin- 

 ance, in short, means that the organism possesses in 

 high degree the inherent powers of reproduction ; and 

 also those activities which enable it to respond by 

 adaptations of morphology, functioning, and behaviour, 

 to environmental changes. These environmental 

 changes are those which must have been experienced 

 during lengthy geological periods, and also those ex- 

 perienced by the organism in its attempt continually 

 to enlarge its area of distribution. 



If we make a broad survey of the animal world we 

 shall find that dominance in these respects has been 

 acquired by three great groups of organisms, (i) the 

 Bacteria, (2) the chlorophyllian organisms- (3) the 

 Arthropods, and (4) the Vertebrates. In each case the 

 threefold condition of wide distribution over all the 

 earth, both in fresh and marine water areas, on the 

 land and in the atmosphere ; of existence throughout 

 the greater part of geological time ; and of ability to 

 withstand environmental change, are satisfied. The 

 bacteria are known to have existed in the carboniferous 

 period. At the present time their distribution on the 

 earth is universal : no part of the land surface, and no 

 water masses, either marine or lacustrine — no matter 

 how unsuitable they may be for the life of more highly 

 organised creatures — are untenanted by bacteria. 

 They are able to withstand extremes of temperature, 

 or of salinity, which would be fatal to the multicellular 

 plant or animal. Parasitism is a mode of life which 

 they exhibit in a more manifold degree than do any 

 other organsims. The upper regions of the atmosphere 



