gical fact, with a fundamental property of
living matter, which governs and includes all its other properties. How
may this property be stated? Thus: it is a property of living matter
to react in a remarkable way to external forces without undergoing
destruction. The life-cycle, of which the embryonic and larval periods
are a part, consists of the orderly interaction between the organism
and its environment. The action of the environment produces certain
morphological changes in the organism. These changes enable the organism
to come into relation with new external forces, to move into what
is practically a new environment, which in its turn produces further
structural changes in the organism. These in their turn enable, indeed
necessitate, the organism to move again into a new environment, and so
the process continues until the structural changes are of such a nature
that the organism is unable to adapt itself to the environment in which
it finds itself. The essential condition of success in this process is
that the organism should always shift into the environment to which its
new structure is suited--any failure in this leading to the impairment
of the organism. In most cases the shifting of the environment is a
very gradual process (whether consisting in the very slight and gradual
alteration in the relation of the embryo as a whole to the egg-shell or
uterine wall, or in the relations of its parts to each other, or in
the successive phases of adult life), and the morphological changes in
connection with each step of it are but slight. But in some cases jumps
are made such as we find in the phenomena known as hatching, birth, and
metamorphosis.

This property of reacting to the environment without undergoing
destruction is, as has been stated, a fundamental property of organisms.
It is impossible to conceive of any matter, to which the term living
could be applied, being without it. And with this property of reacting
to the environment goes the further property of undergoing a change
which alters the relation of the organism to the old environment
and places it in a new environment. If this reasoning is correct, it
necessarily follows that this property must have been possessed by
living matter at its first appearance on the earth. In other words
living matter must always have presented a life-cycle, and the question
arises what kind of modification has that cycle undergone? Has it
increased or diminished in duratio