THE FACTORS OF ORGANIC EVOLUTION. 37 



should, I think, be regarded as at least a good working 

 hypothesis. 



But now supposing the broad conclusion above drawn to 

 be granted supposing all to agree that from the beginning, 

 along with inheritance of useful variations fortuitously 

 arising, there has been inheritance of effects produced by 

 use and disuse; do there remain no classes of organic 

 phenomena unaccounted for ? To this question I think it 

 must be replied that there do remain classes of organic 

 phenomena unaccounted for. It may, I believe, be shown 

 that certain cardinal traits of animals and plants at large 

 are still unexplained; and that a further factor must 

 be recognized. To show this, however, will require 

 another paper. 



II. 



Ask a plumber who is repairing your pump, how the 

 water is raised in it, and he replies &quot; By suction.&quot; Recall 

 ing the ability which he has to suck up water into his 

 mouth through a tube, he is certain that he understands 

 the pump s action. To inquire what he means by suction, 

 seems to him absurd. He says you know as well as he 

 does, what he means ; and he cannot see that there is any 

 need for asking how it happens that the water rises in the 

 tube when he strains his mouth in a particular way. To 

 the question why the pump, acting by suction, will not 

 make the water rise above 32 feet, and practically not so 

 much, he can give no answer; but this does not shake his 

 confidence in his explanation. 



On the other hand an inquirer who insists on knowing 

 what suction is, may obtain from the physicist answers 

 which give him clear ideas, not only about it but about 

 many other things. He learns that on ourselves and all 



