In the ordinary course of nature, all living matter proceeds from
pre-existing living matter, a portion of the latter being detached and
acquiring an independent existence. The new form takes on the characters
of that from which it arose; exhibits the same power of propagating
itself by means of an offshoot; and, sooner or later, like its
predecessor, ceases to live, and is resolved into more highly oxidated
compounds of its elements.
Thus an individual living body is not only constantly changing its
substance, but its size and form are undergoing continual modifications,
the end of which is the death and decay of that individual; the
continuation of the kind being secured by the detachment of portions
which tend to run through the same cycle of forms as the parent. No
forms of matter which are either not living, or have not been derived
from living matter, exhibit these three properties, nor any approach to
the remarkable phenomena defined under the second and third heads. But
in addition to these distinctive characters, living matter has some
other peculiarities, the chief of which are the dependence of all its
activities upon moisture and upon heat, within a limited range of
temperature, and the fact that it usually possesses a certain structure
or organization.
Life conditioned by moisture.
As has been said, a large proportion of water enters into the
composition of all living matter; a certain amount of drying arrests
vital activity, and the complete abstraction of this water is absolutely
incompatible with either actual or potential life. But many of the
simpler forms of life may undergo desiccation to such an extent as to
arrest their vital manifestations and convert them into the semblance of
not-living matter, and yet remain potentially alive. That is to say, on
being duly moistened they return to life again. And this revivification
may take place after months, or even years, of arrested life.
Life conditioned by temperature.
The properties of living matter are intimately related to temperature.
Not only does exposure to heat sufficient to coagulate protein matter
destroy life, by demolishing the molecular structure upon which life
depends; but all vital activity, all phenomena of nutritive growth,
movement and reproduction are possible only between certain limits of
temperature. These limits may be set down as from a little above the
freezing point of water to a little below the boiling point It is to be
noted, however, that these limits apply to the living matter itself, and
many of the apparent exceptions are due to cases in which the living
matter is enclosed in protective wrappings capable of resisting heat and
cold. In many low organisms, such as the spores of bacteria, the thick,
non-conducting wall may preserve the living protoplasm from subjection
to external temperatures below freezing point, or above boiling point,
but all the evidence goes to show that applications of such cold or
heat, if prolonged or arranged so as to penetrate to the living matter,
destroy life. In warm-blooded animals, such as birds and mammals,
protective mechanisms for the regulation of temperature enable them to
endure exposure to extreme heat or cold, but in such cases the actually
living cells do not appreciably rise or fall in temperature. A variation
of a very few degrees in the blood itself produces death.
Recent investigations point to the conclusion that the immediate cause
of the arrest of vitality, in the first place, and of its destruction,
in the second, is the coagulation of certain substances in the
protoplasm, and that the latter contains various coagulable matters,
which solidify at different temperatures. And it remains to be seen, how
far the death of any form of living matter, at a given temperature,
depends on the destruction of its fundamental substance at that heat,
and how far death is brought about by the coagulation of merely
accessory compounds.
Life and organization.
It may be safely said of all those living things which are large enough
to enable us to trust the evidence of microscopes, that they are
heterogeneous optically, and that their different parts, and especially
the surface layer, as contrasted with the interior, differ physically
and chemically; while, in most living things, mere heterogeneity is
exchanged for a definite structure, whereby the body is distinguished
into visibly different parts, which possess different powers or
functions. Living things which present this visible structure are said
to be _organized_; and so widely does organization obtain among living
beings, that _organized_ and _living_ are not unfrequently used as if
they were terms of co-extensive applicability. This, however, is not
exactly accurate, if it be thereby implied that all living things have a
visible organization, as there are numerous forms of living matter of
which it cannot properly be said that they possess either a definite
structure or permanently specialized organs: though, doubtless, the
simplest particle of living matter must possess a highly complex
molecular structure, which is far beyond the reach of vision.
The broad distinctions which, as a matter of fact, exist between every
known form of living substance and every other component of the material
world, justify the separation of the biological sciences from all
others. But it must not be supposed that the differences between living
and not-living matter are such as to justify the assumption that the
forces at work in the one are different from those which are to be met
with in the other. Considered apart from the phenomena of consciousness,
the phenomena of life are all dependent upon the working of the same
physical and chemical forces as those which are active in the rest of
the world. It may be convenient to use the terms "vitality" and "vital
force" to denote the causes of certain great groups of natural
operations, as we employ the names of "electricity" and "electrical
force" to denote others; but it ceases to be proper to do so, if such a
name implies the absurd assumption that "electricity" and "vitality" are
entities playing the part of efficient causes of electrical or vital
phenomena. A mass of living protoplasm is simply a molecular machine of
great complexity, the total results of the working of which, or its
vital phenomena, depend--on the one hand, upon its construction, and,
on the other, upon the energy supplied to it; and to speak of "vitality"
as anything but the name of a series of operations is as if one should
talk of the "horologity" of a clock.
Classification of the phenomena of life.
Living matter, or protoplasm and the products of its metamorphosis, may
be regarded under four aspects:--
1. It has a certain external and internal form, the latter being more
usually called structure;