258 THE PHILOSOPHY OF BIOLOGY 



as to which of the great subdivisions of life repre- 

 sents the main results of the evolution of complex 

 organic entities from the simple living substances in 

 which we suppose life first became materialised on 

 our earth. What activities and structural forms 

 represent the main manifestations of the evolution- 

 ary process ? 



That is to say, what great groups of organisms are 

 the dominant ones on the earth ? Greater or less 

 degrees of dominance are indicated by the extent to 

 which a group of organisms is distributed on the earth, 

 by its abundance, and by the period of time during 

 which it can be recognised in the fossil condition. 

 Ubiquitous distribution implies a high degree of adapta- 

 bility : a group of organisms inhabiting land and sea 

 and atmosphere is obviously one in which the morpho- 

 logical structure has been elastic enough to admit of 

 the development of various modes of locomotion ; 

 and the limbs may be either the appendages of a 

 terrestrial animal, or the fins, or other swimming organs, 

 of an aquatic creature, or the wings of one adapted 

 for flight. Dominance in this respect implies mobility 

 and activity, and a relatively highly developed nervous 

 system ; it implies the development of organs special- 

 ised for prehension, that is, for the capture of food ; 

 and it also implies a high degree of adaptability to 

 widely different physical conditions, to temperature 

 changes, for instance. Dominance in geological time 

 means also this great adaptability to changes in climatic 

 conditions, and the development of means of distribu- 

 tion sufficient to overcome extensive physical changes 

 on the surface of the earth. A terrestrial species might 

 become isolated by the formation of a mountain range, 

 or the submergence of the land adjacent to that which 

 it inhabited, and some widely distributed species of 



