Organisms Consistivg of One Cell 247 



bles, and its organ must be such as to admit the action of 

 that kind or order," and went on to point out that the 

 organs must be "heterogeneous," (that is, as later anatomy 

 came to say, composed of tissues); and when Wihiam Har- 

 vey spoke of the heart as an organ and described its shape, 

 structure, blood capacity, density and movements, and the 

 passage of the blood through it, beyond a shadow of doubt 

 these men had a perfectly clear conception of the most es- 

 sential attributes of an organ, though of course they knew 

 nothing about cells. And does any one believe that had 

 they seen the locomotor parts, let us say, or the mouth of 

 Diplodinium, they would have hesitated, though still whollv 

 ignorant of cells, to call these parts organs, doing so on 

 the same basis on which they had called the sensory parts 

 and the heart of larger animals organs? Can any one 

 either fail to see the point or refuse to admit the validity 

 of the argument? 



The term organ stands for certain kinds of natural ob- 

 jects. First and foremost these objects are definite parts 

 of an organism ; definite that is, in that they have a certain 

 form and character of their own, perform definite offices in 

 the economy of the organism as a totality, and are in turn 

 composed of definite elements. Thus understood the term 

 has had a place in the science of living beings for centuries, 

 as we have just seen. How now, has the advance of knowl- 

 edge affected the earlier understanding of the nature of or- 

 gans? Undoubtedly it has expanded that understanding 

 in a number of directions ; it has made it fuller. One of tlie 

 directions of enlargement pertains to the composition of the 

 organs. It is found that for a large part of the organic 

 world, namely the ?ww/ficellular animals and ])laiits, the ele- 

 ments of the organs are tissues, all of which are derived from 

 still another kind of elements, namely cells. 



The discovery of cells has greatly enriched and clarified 

 the definition of the term organ ; but there is no shadow of 



