Reaction and Environment 177 
my present purpose to penetrate beyond those precincts into the 
realms of philosophy. We have to do with an ultimate biological 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 necessi- 
tate, 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 impair- 
ment 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 duration and complexity since organisms first appeared 
on the earth? The current view is that the cycle was at first very 
short and that it has increased in length by the evolutionary creation 
of new adult phases, that these new phases are in addition to those 
already existing and that each of them as it appears takes over from 
the preceding adult phase the functional condition of the reproductive 
D. 12 
