D.—ZOOLOGY gI 
quite unknown in the physical world. It is important to notice that this 
complexity of structure is essentially of a dynamic nature. We may say, 
if we feel disposed, that it is a system which is physically unstable—but 
where in the chemical world do we find such unstable systems acting in 
such a way as to build up and not to break down a highly complex 
structure ? 
It must be noted that the organising centres of the egg possess physical 
properties by virtue of which their activity may be influenced by external 
conditions. The development of a frog’s egg is affected in a definite 
way by a gradient of temperature applied along particular axes, and we 
know that the egg will not develop in the absence of atmospheric oxygen. 
Can we not say with equal truth that the production of a motor car would 
also be affected by keeping one end of the factory at 30° C. and the other 
at o° C.? Would it not also be affected by depriving the system of 
atmospheric oxygen? ‘The effect of such conditions can be measured 
in terms of physical chemistry, but do they throw any real light on 
the type of organisation necessary for the production of a car or of an 
organism ? 
Within the sphere of embryology we can recognise, more clearly than 
in any other biological science, the two main attributes of living matter : 
(1) an inherent complexity of structure, and (2) a dynamic potentiality 
of initiating events which either do not occur at all or only occur very 
infrequently in inanimate systems. 
Similar inferences can be drawn from another great sphere of experi- 
mental inquiry—namely, a study of the relationship between the fully 
grown organism and its physical and chemical environment, but in this 
case we tend to concentrate on the physical events rather than on the 
potentiality of the organism to control or vary its own activities. For 
example, many animals have the power to elaborate a peculiarly beautiful 
chemical machine for the conveyance of oxygen to the tissues. In such 
cases our main objective is a description of the physico-chemical properties 
of such respiratory systems, and as these are clearly susceptible to statistical 
treatment they can be described in terms of known physical laws. So 
also, in the adult animal, the phenomena of co-ordinated behaviour are 
clearly associated with the central nervous system, and the physical signs 
of this co-ordination are rapidly being analysed by appropriate physical 
methods, but it is important to remember that the phenomena of regulative 
control are present long before the central nervous system has been fully 
differentiated, and are not infrequently detectible in the undivided egg. 
If we are fully to understand the mechanisms of respiration and of co- 
ordinated behaviour we must bear in mind the manner in which the 
fully formed systems come into existence, and not concentrate exclusively 
on the more obvious physical characteristics of the fully developed 
mechanisms. 
Let us now try to summarise the position. The only laws which 
physics has provided for an analysis of biological phenomena rest on a 
Statistical basis ; they only apply to systems which contain a large number 
of participating units and only describe natural phenomena in terms of 
probability and not of absolute truth. If we accept these laws as a means 
of describing the behaviour or the structure of an organism, we must 
