Preface. vii 



It has always been the chief hindrance to a more rapid 

 advance in botany, that the majority of writers simply 

 collected facts, or if they attempted to apply them to 

 theoretical purposes, did so very imperfectly. I have 

 therefore singled out those men as the true heroes of 

 our story who not only established new facts, but gave 

 birth to fruitful thoughts and made a speculative use of 

 empirical material. From this point of view I have taken 

 ideas only incidentally thrown out for nothing more than 

 they were originally ; for scientific merit belongs only to 

 the man who clearly recognises the theoretical importance 

 of an idea, and endeavours to make use of it for the pro- 

 motion of his science. For this reason I ascribe little 

 value, for instance, to certain utterances of earlier writers, 

 whom it is the fashion at present to put forward as the 

 first founders of the theory of descent ; for it is an in- 

 dubitable fact that the theory of descent had no scientific 

 value before the appearance of Darwin's book in 1859, and 

 that it was Darwin who gave it that value. Here, as in 

 other cases, it appears to me only true and just to abstain 

 from assigning to earlier writers merits to which prob- 

 ably, if they were alive, they would themselves lay no 

 claim. 



J. SACHS. 



\ViJRZBURG, Jtlly 22, 1875. 



