D.—ZOOLOGY 85 
expected it to occur must be slightly demented. In the case of the 
Brownian particles, the chances of ten contiguous particles moving 
simultaneously in the same direction are even smaller, and in practice we 
sum all this up by saying that as long as we are dealing with reasonably 
large numbers of molecules, the events which we observe are the most 
probable events, and we assume that the improbable events do not in 
fact occur. On this arbitrary but effective basis rest most, if not all, the 
laws of physics and chemistry which we apply to the study of living matter. 
We say, in effect, that stones do not leap spontaneously from the earth 
because the chances against it are so extremely great ; similarly we state 
that the pressure of a gas is always inversely proportional to its volume, 
except on a negligible number of occasions. ‘The organisation of the 
simplest living organism is clearly more complex than that of a stone or of 
a motor car, and it carries out processes which are infinitely more complex 
than the sorting out of black from white particles. What, in fact, is the 
probability that any chance distribution of molecules should lead spon- 
taneously to the dynamically active mechanism of the living organism ? 
Would any serious credence be given to the suggestion that a motor car 
or even a footprint on the sands came spontaneously into existence without 
the intervention of directive forces? Why, then, should we accept the 
spontaneous origin of living matter ? It is possible, but it is so improbable 
that, if considered as an observable phenomenon, in any other sphere of 
human thought it would be discarded as a figment of a deranged brain. 
Why should biology accept a standard of probabilities incomparably less 
satisfying than that of other branches of knowledge ? 
Left to himself, the chemist does not seriously consider the spontaneous 
origin of proteins from CO,, water, and simple salts, nor does the physicist 
admit the spontaneous origin of organised machines. Biology itself pro- 
vides not one shred of observational evidence to support the spontaneous 
origin of living matter in the world to-day, and yet not a few biologists 
are prepared to postulate the spontaneous origin of intermediate stages 
between the living and the inanimate worlds—to my mind, the spontaneous 
origins of these stages represent physical events which are so improbable 
that we cannot describe them in terms of ‘ laws’ which only apply to 
events of an entirely different order of probability: if these inter- 
mediate stages actually occurred they must be classified as miracles, 
not as ‘ natural’ events. We may be told that in past ages, events which 
are now very improbable were in fact of quite frequent occurrence. As 
scientists we cannot accept this statement without some assurance as to 
what were the nature of the conditions which made the origin of life 
inevitable or even probable. The distribution of energy and of matter 
in past epochs may have been different ; but if such conditions produced 
the living organism, is it not strange that every attempt to reproduce 
them in the laboratory have completely failed ? 
We can put the facts in another way. Within the physical world all 
systems appear to move towards the state of greatest probability, and the 
events which take place within a dynamic system are those which tend 
to destroy structure and not those which elaborate it. Is there any evi- 
dence which suggests that, within the physical world, a dynamic machine 
has spontaneously come into existence? ‘That such an event might 
