i68 
C.W. Soal 
II. It is necessary now to investigate further the relation between 
the sequences of biological events which we have termed organic 
activities, and the energy-reactions with which they are invariably 
associated. Now whether the former are, or are not, capable of an 
ultimate interpretation in physiological terms, there is, abundant 
evidence that they are always limited by the intrinsic physical 
properties of the primary and secondary environment. In general 
terms it may be said that throughout development the organism is 
always conditioned by the persistence of certain physical equilibria 
between its various reactions. We shall deal first with the operation 
of limiting factors in the secondary environment. 
It is possible to ascertain some of the proximate secondary factors 
in somatic equilibrium owing to the persistence of certain types of 
reaction as ‘'structure and function” through various periods of 
development, our present knowledge being most extensive in the 
case of adult organisms. The internal or physiological reactions 
always include some of the most important factors, which are also 
usually more fundamental from the phylogenetic point of view. The 
life of an adult mammal, for instance, is conditioned by the fact 
that it has acquired somehow, in the course of individual and racial 
development, a type of internal economy involving the interaction 
of various structural characters such as the nervous, vascular and 
glandular systems that remain relatively constant during an appreci¬ 
able period. If any of these characters were changed, in some cases 
even in certain minute details ( e.g . the chemical function of the 
erythrocytes), the animal would die unless there were at the same 
time some compensative readjustment in its physiological economy. 
The continued stability of the organism throughout ontogeny is 
dependent upon the fact that the changes which do take place in 
these physiologically important characters are mutually compensative 
in this respect. It must be carefully noted however, that these 
physical equilibria conditions do not actually determine the sequence 
of biological events, which must still be ascribed to a particular 
quality of the activity-system; they merely impose certain limita¬ 
tions within which possible sequences may occur 1 . 
In the higher types somatic equilibrium may be also largely con¬ 
ditioned by environmental reactions external to the body of the 
organism, as for instance by the efficiency of the various mechanical 
1 In the case of plants the proximate secondary environmental limiting 
factors are not so obvious, but they are present none the less; the function of 
the chloroplasts and the structure of the vascular system may be mentioned. 
