C. W. SOAL 
184 
the only subsequent variations will be sporadic and adaptational. 
The continued existence of representatives of archaic forms, such as 
unicellular algae and protozoa, liverworts, mosses, sponges, corals, 
etc., must be ascribed to this fact. It is important to note that from 
our point of view the relative fixity of these forms, which may be 
compared to truncated lateral branches in the evolutionary tree, is 
not due to any peculiar quality of the germ-plasm, but is an intrinsic 
physical limitation imposed by the type of physiological economy 
they have developed. A marked further increase in efficiency could 
only result from some fundamental change in organisation, which is 
inhibited by the essential stability of the more important structures 
and functions. They may however still undergo considerable changes 
in relation to environmental conditions, while retaining the same 
general type. The divergent trends of variation that have led to the 
more efficient types must therefore have been initiated in less 
specialised forms. The inferior stable types are those which failed to 
take the right turning at some critical period in their phylogenetic 
history, when a deflection of the energy of the organism into alter¬ 
native channels was possible. 
From the biological standpoint, the evolution of the higher 
animals is characterised, as we have seen, by the synthesis of sporadic 
and ephemeral reactions with the primary environment into complex 
activities, which in turn become further integrated with the more 
stable activity-system of the organism, subject of course at each step 
to the limitations imposed by the environmental factors involved. 
In the higher types a modification of instinctive or intelligent 
behaviour is one of the most powerful means of increasing physio¬ 
logical efficiency, and it also permits the further integration of the 
whole activity-systems of different individuals into the still more 
efficient social groups. 
From the energy standpoint, this process of progressive integration 
is represented by an increasing complexity in the secondary environ¬ 
ment of the organism. In plants the energy-reactions which are 
co-ordinated in somatic development are practically all internal or 
physiological in character. The higher animal types on the other 
hand have acquired a capacity, as it were, for reaching out into the 
primary environment far beyond the limits of their morphological 
bodies, and progressively incorporating diverse external reactions 
within the sphere of their economic system. This “ external physi¬ 
ology” is very varied. In its simplest form it is represented by 
various supplementary mechanical devices, such as cells, nests, etc., 
