CONCLUSION 171 



advances which have been made during the past 

 hundred years would not have been so consider- 

 able. If the hypothesis is constructed after the 

 work is done or before it is completed, the work 

 accomplished remains after the need for the 

 hypothesis, which is either the best clue that 

 one can give for the time being or the best 

 guide that one can follow, has passed away. 

 In morphology everything is important except 

 the hypothesis, although practically nothing could 

 be done without it, since it is often the only 

 means available for digesting an accumulation 

 of facts. It is something intangibly necessary, 

 often quite wrong, always hopelessly incom- 

 plete, but ever ready to give way by substitution 

 to another invisible vehicle. The progress of 

 morphology depends upon the substitution of 

 ideas rather than upon the promulgation of laws. 

 The tree of life is polyphyletic, and the branches 

 do not anastomose after their zigzag course has 

 been set. 



