or even to the smallest animalcule
or unicellular plant is always partly fluid, but never entirely so.
Every living creature also consists in part (and that part is the most
active living part) of a soft, viscid, transparent, colorless
substance, termed protoplasm, which can be resolved into the four
elements, oxygen, hydrogen, nitrogen and carbon. Besides these four
elements, living organisms commonly contain sulphur, phosphorus,
chlorine, potassium, sodium, calcium, magnesium and iron.

In the fact that living creatures always consist of the four elements,
oxygen, hydrogen, nitrogen and carbon, we have a fundamental character
whereby the organic and inorganic (or non-living) worlds are to be
distinguished, for as we have seen, inorganic bodies, instead of being
thus uniformly constituted, may consist of the most diverse elements
and sometimes of but two or even of only one.

Again, many minerals, such as crystals, are bounded by plain surfaces,
and, with very few exceptions (spathic and hematite iron and dolomite
are such exceptions) none are bounded by curved lines and surfaces,
while living organisms are bounded by such lines and surfaces.

Yet, again, if a crystal be cut through, its internal structure will
be seen to be similar throughout. But if the body of any living
creature be divided, it will, at the very least, be seen to consist of
a variety of minute distinct particles, called "granules," variously
distributed throughout its interior.

All organisms consist either--as do the simplest, mostly microscopic,
plants and animals--of a single minute mass of protoplasm, or of a
few, or of many, or of an enormous aggregation of such before-mentioned
particles, each of which is one of those bodies named a "cell" (Fig.
3). Cells may, or may not, be enclosed in an investing coat or
"cell-wall." Every cell generally contains within it a denser,
normally spheroidal, body known as the nucleus.

Now protoplasm is a very unstable substance--as we have seen many
substances are whereof nitrogen is a component part--and it possesses
active properties which are not present in the non-living, or
inorganic world. In the latter, differences of temperature will
produce motion in the shape of "currents," as we have seen with
respect to masses of air and water. But in a portion of protoplasm,
an internal circulation of currents in definite lines will establish
itself from other causes.

Inorganic bodies, as we have seen, will