78 SECTIONAL ADDRESSES. 
So far observations and tests have shown that new characters due to 
modification only reappear so long as the new stimulus persists. The 
difference lies not in the value or permanence of the new character, but 
in the causes which give rise to it.° 
It is little more than a platitude to state that, for the production of 
an organism or of any of its characters, both germinal factors and 
environmental stimuli are necessary, and that if evolution is to take 
place there must be change in one or both. Yet the changes in the 
factors may be held to be the more important. . In an environment 
which on the whole alters but little, evolution progresses by the cumu- 
lation along diverging lines of adaptation of new characters due to 
mutation. Thus natural selection indirectly preserves those factorial 
complexes which respond in a favourable manner. In other words, 
an organism, to survive in the struggle for existence, must present that 
assemblage of factors of inheritance which, under the existing environ- 
mental conditions, will give rise to advantageous characters. 
In answer to a further question, let us now try to explain what we 
mean when we contrast the organism with its environment. In its 
simplest and most abstract form a living organism may be hkened to 
a vortex. That mixture of highly complex proteins we call protoplasm, 
the physical basis of life, is perpetually undergoing transformations of 
matter and energy, so long as life persists. Towards the centre of the 
vortex the highest compounds are continually being built up and. con- 
tinually being broken down; new material (food, water, oxygen) and 
energy are brought in at the periphery, and old material and energy 
(work and heat) thrown out. ‘The principle of the conservation of 
energy and matter holds good in organised living processes as it does 
in the inorganic world outside. This is the process we call metabolism, 
and it is at the base of all the manifestations of life. From the point 
of view of biological science life is founded on a complex and continuous 
physico-chemical process of endless duration so long as conditions are 
favourable; just as a fire will continue to burn so long as fuel is at 
hand. No one step, no single substance, can be said to be living: the 
whole chain of substances and reactions, every link of which is essential, 
constitutes the life-process. A stream of non-living matter with stored- 
up energy is built up into the living vortex, and again passes out as 
dead matter, having yielded up the energy necessary for the performance 
of the various activities of the organism. If more is taken in than is 
given out it will grow and sub-divide. The complexity of the organism 
may increase by the formation of subsidiary, more or less interdepen- 
dent, vortices within it. The perpetual growth and transmission of 
factors of inheritance, the continuity of the germ-plastm, is but another 
aspect of the continuity of the metabolic process forming the basis of 
the continuity of life in evolution. 
5 We might perhaps distinguish the two cases by calling them constant 
and inconstant characters, or ‘natural’ and ‘ acquired,’ as is commonly done 
when describing immunity. It should be meant thereby that one is acquired 
usually (under normal conditions), the other occasionally (when infection 
occurs). Error creeps in when the term ‘acquired’ is opposed to ‘ non- 
acquired ’ or to ‘ inherited.’ 
