296 Examination of the Matured Framework [BOOK n. 



monograph of this kind was a nucleus, round which a larger 

 number of observations might afterwards gather. In a long 

 series of such solid productions he treated conclusively all the 

 more important questions of phytotomy. 



Von Mohl's extraordinary carefulness was not however able to 

 guard him, calm observer though he was, from some serious 

 mistakes, at least in his earlier years, such as those which occur 

 in his first theory of intercellular substance (1836), and in his 

 earliest views on the nature of the cell-membrane of the pollen- 

 grain (1834). These and some other errors on the part of 

 a gifted and truly inductive enquirer are instructive, since they 

 show that observation without any ground-work of theory is 

 psychologically impossible ; it is a delusion to suppose that an 

 observer can take the phenomena into himself as photographic 

 paper takes the picture ; the sense-perception encounters views 

 already formed by the observer, preconceived opinions with 

 which the perception involuntarily associates itself. The only 

 means of escaping errors thus produced lies in having a distinct 

 consciousness of these prepossessions, testing their logical 

 applicability and distinctly defining them. When von Mohl 

 laid down his theory of intercellular substance, there evidently 

 floated before his mind indistinct, half-conscious ideas of the 

 kind that Wolff and Mirbel entertained of the structure of the 

 vegetable cell ; and as he considered the cell-membrane of the 

 pollen-grain to consist of a cell-layer, he summarised its obscure 

 structural relations under the then very obscure conception of 

 the cell. As a true investigator of nature, who adheres always 

 and firmly to the results of further observation, and endeavours 

 to clear his ideas by their aid, conceding only a relative value to 

 every view, von Mohl soon escaped from these errors, and him- 

 self supplied proofs of the incorrectness of his former opinion. 

 The number of really erroneous statements in his works is 

 wonderfully small considering the very large number of investi- 

 gations in which he engaged. 



In examining the part which von Mohl played in the general 



