310 THE AMERICAN MONTHLY. [Oct. 



would recognize at once that the differences so much 

 insisted upon represented different phases of activity, 

 and, as with the pancreatic cells, might be all repre- 

 sented in the same animal at different times. 



It would be far from saying that there are no structural 

 differences in the different animals independent of any 

 particular phase of functional activity ; but if these only 

 are sought and the others neglected, the physiological 

 appearances will often obtrude and confuse, if they do 

 not utterly confound. 



I have therefore for the last ten years urged my stu- 

 dents and mean to go on advocating with all the earnest- 

 ness of which I am capable, that, in studying an organ- 

 ism or its tissues, the investigator, to gain certain 

 knowledge, must know all that it is possible to learn 

 concerning the age, health, state of nervous, muscular 

 and digestive activity ; in fact, all that is possible to find 

 out about the processes of life that are going on when 

 the study is made. 



Fortunately, there are some microscopic forms in 

 which the entire study can be made while the creature 

 is alive. With the higher org'anisms also some of the 

 living elements, as the white corpuscles, can be studied 

 and their various actions and structural changes observed 

 for a considerable time. Most of the tissues of the 

 higher forms, however, cannot be thus studied, and the 

 best that can be done is to fix the different phases of 

 action, as by a series of instantaneous photograx)hs, then 

 with a kind of mental kinetoscope put these together and 

 try to comprehend the whole cycle. 



Fortunately for the histologist, the incessant experi- 

 mentation of the last twenty-five years has brought to 

 knowledge chemical substances which do for the tissues 

 the wonder that was ascribed to the mythical Grorgon's 

 head — to kill instantly and to harden into changeless 

 permanence all that gazed upon it. So the tissues may 



