4 PREFACE BY TRANSLATOR. 



which have proved so attractive, especially for the Ger- 

 man 1 and the French savants, there is nevertheless a 

 wide-spread interest in these investigations, and a desire 

 to know their results. But, just here, we are met with 

 a difficulty which has no doubt discouraged many, and 

 perhaps caused some to drop the whole subject in dis- 

 gust. The results have been so contradictory, and so 

 many would-be savants have uttered opinions entirely 

 opposed the one to the other, that we find it impossible 

 to arrive at any definite opinion, not knowing whom to 

 believe. This being the condition of affairs, it seems to 

 me that it is necessary for us to commence investigating 

 for ourselves, — first making ourselves familiar with what 

 has been done abroad, and then avoiding, if possible, the 

 quicksands into which unfortunate science has too often 

 been dragged by her votaries. One great trouble 

 which we have experienced in this country is in judg- 

 ing of the comparative value of the observations of dif- 

 ferent men who are equally unknown to us. A very 

 plausible article may be written by a very careless 

 observer ; or a very cautious observer may fail to give 

 confidence in his results, because of a certain degree of 

 confusion in his language. When experiments are well 

 devised, carefully executed, and described with preci- 

 sion, as is done by such men as Pasteur and Tyndall, we 

 cannot fail to attach great weight to the conclusions 

 reached. And when so accomplished a microscopist as 

 Cohn or Koch asserts that he has seen such and such a 

 thing, or has made such and such measurements, we 

 cannot doubt the reliability of the observation. But 

 sometimes we are deceived by giving credence to a man 

 who has achieved reputation in one line of study, but of 



