150 EVOLUTION AND GENETICS 



long would be in proportion to the length of the 

 trunks already present and to the number of indi- 

 viduals in which such a character might appear. In 

 other words, evolution once begun in a given direc- 

 tion is in a favorable position to go on in the same 

 direction rather than in another {fig. 64) , so long as 



Fig. 64. — Evolution of elephant's trunk. (After Lull.) 



the advance does not overstep the limit where fur- 

 ther change is advantageous. 



The duality of the evolution process from the 

 point of view of natural selection has not always 

 been sufficiently emphasized. A series of events that 

 can be given a stricth^ causal interpretation leads 



