THE FACTORS OF ORGANIC EVOLUTION. 103 
those named above is practically to make impossible at 
present any explanation of vital phenomena. “If we 
would advance without interruption,” says Roux,* “we 
must be content, for many years to come, with an analy- 
sis into complex components,” 
2. We need not now further concern ourselves with 
an explanation of extrinsic causes or simple components, 
since this subject properly belongs to 
chemistry and physics. If, however, we 
examine more closely some of the zz- 
trinsic causes or complex components, we 
will find that they are always associated with more or 
less complex structures ; in fact, they are dependent upon 
structure, 
The smallest and simplest mass of protoplasm that 
can manifest all the fundamental phenomena of life, 
such as assimilation, growth, division, and metabolism, 
is an entire cell, nucleus and cytoplasm, and probably 
centrosome. The cell is composed, as microscopic study 
plainly reveals, of many dissimilar but perfectly co- 
adapted parts, each performing its specific function, and 
it may therefore properly be called an organism. Some 
phenomena of cell life may be directly referred to 
the various visible constituents of the cell, but many of 
them are evidently connected with structures which we 
can not see, structures which may perhaps never be 
seen, and yet which must be vastly more complex than 
the most complex molecules known to chemistry, and 
yet much more simple than the microsomes, centro- 
somes, and chromosomes which are visible in the cell. 
With these ultra-microscopical particles many of the 
most fundamental phenomena of life are associated— 
viz., assimilation, growth, metabolism, and probably 
Intrinsic causes 
arise from nature 
of protoplasm. 
; * Wilhelm Roux. Einleitung: Archiv fiir Entwickelungsmecha- 
nik der Organism. 
