340 _ A Lesson in Comparative Histology. [June, 
changing anything but its size. In living matter, however, nature 
presents an exception to the universality of this law; for in living 
bodies the molecules, which are far smaller than the smallest 
visible particles, are united to form masses of limited size, which 
represent so many units of the body, just as we might say the 
bricks represent so many units of a wall. These units of life, as 
I may call them at the risk of being misunderstood, are the cells 
before mentioned. The great material difference, therefore, 
between living and unorganized bodies was at once demonstrated 
by the discoveries of Schleiden and Schwann, and it is on this 
account that naturalists attribute such importance to the work of 
these two men. In truth Schwann’s investigations caused as 
great a change in the direction pursued by zodlogists in their 
researches as the reform either of Linnaeus or Cuvier. But 
Schwann unlike his great predecessors did not continually make 
further discoveries, and has not, as far as I am aware, participated 
in the work of original research, which has been in progress 
during his life time, so that to most of us perhaps he already 
seems a person of the distant past. 
Yet during Schwann’s lifetime one of the principal labors of 
zodlogists has been the working out in detail the applications of 
his generalization, and determining the variations and modifications 
which cells undergo in the different tissues and species of animals, 
until finally the subject has assumed an importance even more — 
vast than could at first have been foreseen. It is in fact hardly an 
exaggeration to say that all our knowledge of animals groups 
itself about the doctrine of cells, as the central factor upon which 
all others depend; and whether we labor as physiologists, embry- 
ologists or anatomists, we are alike forced ultimately to base all 
our conclusions, and demonstrate all our theorems by the char- 
acter and property of cells. In brief, what a knowledge of waves a 
is to the student of sound, a knowledge of cells is to the student 
of life. 
_ various bearings of the cell doctrine, for we should deal with 
many of the fundamental problems of zodlogy, and with some 
of the most interesting additions to our knowledge in this depart- 
ment of science which have ever been made. This course would — 
take us away from the real subject of this article, the object of : 
which is to give an example of how much may be learned by 
It would be a pleasant task to expatiate at length upon the a 
