16 THE STORY OF FISH LIFE. 



epochs one particular type will have the ascend- 

 ency over all the others, this he calls the "domi- 

 nant" type. He then proceeds to show that 

 a dominant old race, at the beginning of its 

 greatest vigour, seems to give origin to a new 

 type, showing some fundamental change; this 

 advanced form then seems to be driven from all 

 the areas w^here the dominant ancestral race 

 reigns supreme, and evolution in the latter be- 

 comes comparatively insignificant. Meanwhile 

 the banished type has acquired great develop- 

 mental energy, and finally it spreads over every 

 habitable region, replacing the more effete race 

 which originally produced it. The period of 

 greatest vigour of the ''dominant old race" 

 represents the flowering out of new species 

 stimulated into being, by the occupation of 

 new territory, the new species developing as a 

 result of adaptation necessary to obtain a hold 

 on this or that particular area. Now, adaptation 

 spells specialisation, and the cessation of the 

 growth of the dominant race after this sudden 

 burst of activity points to inability to further 

 development, a balance being struck between 

 the organism and the environment. The banished 

 form which suddenly springs up in force replacing 

 the parent type is also the result of adaptation. 

 Born of members of the parent form, but possibly 

 far removed from the environment which was 

 slowly shaping the typical dominant forms, they 

 developed along the new lines demanded by the 

 new environment, which eventually appears to 

 have slowly replaced the old order of things and 

 the highly specialised forms dependent thereon. 



