202 THEORY OF THE CELLS. 
But greatly as these plastic phenomena differ in cells and 
in crystals, the metabolic are yet more different, or rather they 
are quite peculiar to cells. For a crystal to grow, it must be 
already present as such in the solution, and some extraneous 
cause must interpose to diminish its solubility. Cells, on the 
contrary, are capable of producing a chemical change in the 
surrounding fluid, of generating matters which had not pre- 
viously existed in it as such, but of which only the elements 
were present in another combination. They therefore require 
no extraneous influence to effect a change of solubility ; for if 
they can produce chemical changes in the surrounding fluid, 
they may also produce such substances as could not be held in 
solution under the existing circumstances, and therefore need 
no external cause of growth. If a crystal be laid in a pretty 
strong solution, of a substance similar even to itself, nothing 
ensues without our interference, or the crystal dissolves com- 
pletely: the fluid must be evaporated for the crystal to in- 
crease. If a cell be laid in a solution of a substance, even 
different from itself, it grows and converts this substance 
without our aid. And this it is from which the process going 
on in the cells (so long as we do not separate it into its several 
acts) obtains that magical character, to which attaches the idea 
of Life. 
From this we perceive how very different are the phenomena 
in the formation of cells and of crystals. Meanwhile, however, 
the points of resemblance between them should not he over- 
looked. They agree in this important point, that solid bodies 
of a certain regular shape are formed in obedience to definite 
laws at the expense of a substance contained im solution in a 
fluid; and the crystal, like the cell, is so far an active and posi- 
tive agent as to cause the substances which are precipitated to 
be deposited on itself, and nowhere else. We must, therefore, 
_attribute to it as well as to the cell a power to attract the sub- 
stance held in solution in the surrounding fluid. It does not 
indeed follow that these two attractive powers, the power of 
crystallization—to give it a brief title—and the plastic power 
of the cells are essentially the same. ‘This could only be ad- 
mitted, if it were proved that both powers acted according to 
the same laws. But this is seen at the first glance to be by 
no means the case: the phenomena in the formation of cells 
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