Origin and Development of the Nervous System 
73 
developing organism is determined by 
the type of symmetry in the gradient 
pattern. 
COMPLEXITY IN DEVELOPMENT 
As the hereditary potentialities of 
all cells of a given organism are believed 
to remain the same, it may seem 
difficult to understand how a merely 
quantitative gradient can bring out 
the complexity of structure and the 
qualitative differentiation of tissues, 
which make their appearance. The 
hereditary constitution, however, is 
necessarily complex. Different ele- 
ments in it may prevail at different 
rates of metabolism. Moreover, the 
gradient pattern while continuous is 
ever changing. The past history of 
cells is thus a factor limiting their 
later development, in addition to 
genetic constitution, position in the 
gradient pattern of the organism at the 
given time, and direct external stimuli. 
The original axis of highest activity, 
in general, maintains its dominance 
over the lower levels in the gradient 
and tends to become even more 
specialized in the transmission of 
stimuli in animals in which the heredi- 
tary potentiality for such specialization 
is present. Thus the region of first 
active growth in the arthropod egg 
becomes the ventral nerve chain and 
the similar region in vertebrates be- 
comes the medullary plate and 
ultimately the brain and spinal cord. 
In case a secondary center of activity 
becomes sufficiently isolated from the 
primary center, a second individual 
tends to be formed about it. Separate 
individuals may result as in the asexual 
reproduction of many lower vertebrates 
or a system with mechanical continuity 
and perhaps transportative coordina- 
tion, between parts, but only slight 
integration in behavior may result as in 
plants, hydroids, tapeworms, etc. 
Finally through a secondary reattain- 
ment of dominance by the primary 
center, a segmental organism may be 
produced as is suggested for annelids, 
anthropods and vertebrates. 
THE SCOPE OF THE BOOK 
This, in a rough way, may bring 
out Professor Child's theory of develop- 
ment to which the greater part of this 
book is devoted. Considerable atten- 
tion is devoted to the nature of 
stimulation and transmission, to evi- 
dences for the existence of metabolic 
gradients in particular organisms, and 
to applications of the theory to special 
problems such as the development of 
the pattern of nerve cells and of the 
nervous system as a whole. Suggestive 
analogies with phases in the organiza- 
tion of human societies are frequently 
presented. 
RELATION TO VITALISM 
The theory, as a mechanistic one, 
seems the very antithesis of such 
vieWS as those set forth by Driesch 
with his quasi intellectual factor, the 
entelechy, as the guiding spirit of 
development, and by Bergsen with his 
elan vital. In a sense, however, there is 
a curious approach. Under Child's 
theory there is complete continuity 
from the reaction of the cell with its 
environment which constitutes the 
primary metabolic gradient, and from 
the later reactions by which the pattern 
of the developing embryo is laid down 
in accordance with the changing 
gradient pattern, to the intellectual 
processes by which the adult organism 
adjusts its relations to the outside 
world. Since awareness is certainly 
associated with the dominating nervous 
activities in the latter case, it seems 
necessary to grant the possibility of its 
presence in the former, unless we wish 
to assume that it is arbitrarily super- 
imposed upon metabolic gradients at a 
certain level of complexity. Moreover 
as a state of consciousness in the higher 
case is certainly closer to reality than 
any impressions which it may make on 
other consciousnesses, the question 
at least seems open as to whether the 
entelechy may not be the reality of 
which the metabolic gradient, however 
much correlated with environmental 
factors, is merely the outward show. 
