THE PRINCIPLES OF BIOLOGY. 
219 
to tlieir environment. An aggregation of them, necessarily 
a minute one at first, would possess still greater mobility, 
and show the first gleam of what we call “vitality.” It 
would be, in fact, a speck of protoplasm, such as we know 
to exist (Monera, Protomyxa), manifesting some of the 
phenomena of life, but as yet possessing no appreciable 
organisation. 
Small as it is, the incident forces must act unequally upon 
the units of which it is composed. Differentiation begins; 
some of them take upon themselves the duty of protecting 
the others, and become modified for that purpose, this being 
obviously an advantage as the life of the aggregate is thereby 
rendered more secure. This aggregate we name a cell , and 
we thus arrive at the stage of the unicellular organisms 
which have been called Protopliyta and Protozoa. Of the 
former Bacteria, Protococcus, and the simpler Diatoms and 
Desmids are progressively more complex examples. 
It will be observed here that no account is taken of the 
development of the cell-nucleus, and it might be supposed 
that without this no explanation of the gradual evolution of 
organisms could be complete. But the fact is that a “ cell- 
nucleus ” (however important may be and is the part which 
it plays in the more highly developed plants) is at first 
nothing more than a slightly condensed portion of the proto¬ 
plasm and exerts hardly any influence upon the life of the 
elementary organisms in which it occurs. We find protoplasm 
with or without a nucleus, and with or without a cell-wall; 
thus giving four states of existence. It may be an advantage 
to restrict the term “ cell ” to those cases where a wall and a 
nucleus are both present, but there is at first no distinction 
of importance between these cells and the “ cytodes ” which 
have a wall, but no nucleus; and in higher organisms many 
structures are loosely called “cells,” which neither have nor 
have had a nucleus. Those nuclei which are now known 
to perform such promine 1 ' md mysterious functions are 
but the highest terms' ' 0 radual series of developments. 
Let us now pause to check our hypothesis. If the mode 
indicated in the previous paragraphs be that in which the 
present organic world began, and if all organisms of higher 
structure be the descendants of ancestors resembling those 
just described, the doctrines of Inheritance and Keversion 
(which, be it remembered, are established by pure observation) 
will lead us to expect two things:—(1) That cells will play a 
conspicuous part in the structure of these higher organisms; 
and (2) that parts of them will be occasionally constructed 
directly from protoplasm that has not been formed previously 
