4 PREFACE BY TRANSLATOR. 
which have proved so attractive, especially for the Ger- 
man 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 sucha 
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 
