138 CLUES AND SUGGESTIONS CHAP. 



Yet nobody would be tempted to guess 

 on this account, except in a metaphorical 

 sense, that steam-engines have all been 

 begotten by each other. The metaphor 

 from organic births, however, is so 

 apposite and perfect in its analogy that 

 it is often actually used, and the beget- 

 ting of ideas, or of the application of 

 ideas to mechanical or chemical work, 

 is a recognised branch of the history of 

 mechanics. 



The truth is that the argument de- 

 rived from the principle on which all 

 natural classifications rest, is a very 

 dangerous argument for Darwinians. 

 It cuts two ways, and one of the ways 

 is very undermining to the assumption 

 that there has been some continual flux 

 of specific characters. It is true that in 

 all living structures common features, so 

 numerous, do indicate some common 

 cause and source. But it is not less 



