448 DIRECTIVE FACTORS IN EVOLUTION: 



tion may operate with great delicacy in reference to a sieve 

 with fine meshes, it may also rough-hew and cut off in a 

 crisis a large number of organisms which are in a general 

 way less fit than their fellows. As Professor Bumpus said, 

 general stability of structure was the essential characteristic 

 of the surviving sparrows (Bumpus, 1898). 



(4) An interesting corollary to the selection proposition 

 is that a relaxation of sifting may admit of exuberance. 

 When organisms reach a position of relative security, as 

 many species do, then, the criticism of circumstances being 

 removed, there may be extraordinary abandon in the way 

 of coloration and decoration. The limit is the stability of 

 the constitution ; the risk is that some environmental change 

 may involve a heavy tax on the exuberance which the condi- 

 tions of relaxed selection tolerated. It may be one reason 

 of the diversified brilliance of humming birds that they have 

 few enemies. A clearer case is to be found in the coral- 

 fishes, whose exuberance of coloration beggars description 

 (see Reighard). It may be that the gorgeousness has been 

 made possible by the safety of the labyrinthine reefs, and 

 by the agility of the swimmers. Prof. J. P. Lotsy (1916) 

 speaks of the bewildering diversity exhibited by a series 

 of about 200 specimens of the Common Buzzard (Buteo 

 'buteo') in the Leiden Museum, " hardly two of which are 

 alike ". " The reason probably is that here no selection has 

 been at work, because this bird of prey is so strong that it. 

 has practically no enemies in the regions in which it occurs." 



(5) Of great importance is the change that has been in- 

 volved in our appreciation of Natural Selection by an in- 

 creased knowledge of the raw materials supplied to the sieve 

 by variability. As we have seen, discontinuous variations 

 or mutations are not of rare occurrence ; there is a brusque 



