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©ope.] 256 [Dee. 15, 
tion through circulation. The nervous system, like others, develops i 
capacity with use, hence probably nerve tissue converts heat into nerve 
force as muscular tissue converts heat into motion. In other words, by 
repetition, the capacity of the nervous system for this conversion of heat 
is known to increase. As the amount of heat converted is in proportion 
to the amount of appropriate nerve tissue (see above) it is evident that 
use and effort increase the amount of nerve tissue. 
The phenomena of thought render the same modification of structure 
probable.  ffort in the direction of thought is supposed to convert heat 
into thought force. Inasmuch as the more intelligent animals possess the 
highest development of cerebral hemispheres, it is highly probable that 
brain substance converts heat into growth force also, which produces tissue 
of its own kind precisely as muscle does. 
As different parts of the nervous centres, subserve different purposes, 
the development of these parts must proceed approximately under the 
influence of special kinds of effort and use. Where, as in the adult, heat 
is converted into growth force in the tissues toa very limited extent, if 
the above principles be true, the conversion of heat by the nervous system 
into nerve growth force and tissue, is on the other hand, not terminated. 
Capacity for effecting conversion of force is regarded, as above pointed 
out, as dependent on molecular constitution. Hence we conclude that 
change in that capacity on the part of the nervous system involves a mo- 
lecular change in its constitution. 
Now, it is apparent that if the nervous centres possessed the enlarged 
capacities for the conversion of heat into nerve force and thus of con- 
stantly controlling the circulation in special directions, in a growing or 
foetal animal, tissue will be produced in the directions in question. For 
the heat converted into motion in the adult is in the foetus in large part 
converted into growth-force. ; 
Now, we know physical and metaphysical peculiarities of parents to be 
inherited by offspring, hence, no doubt, the nervous structure determina- 
tive of growth force is inherited. This will then control the localities of 
special conversion of heat, ete., (from the mother) into growth force, in 
accordance with the structure of the parent, and the more decidedly, as 
its own increase progresses. 
The result will be acceleration, or construction of tissues and organs in 
excess of those of the parent, if the effort or use devoted to a nerve or 
organ be represented in the nerve centre of the parent by a greater amount 
of force-converting tissue, than is necessary when iuherited in the fetus 
for the construction (by conversion), of tissues and organs like those of 
the parent. 
That this is a partial explanation of inheritance, is rendered probable 
from the fact that, the types of structure presented by the nervous centres, 
express the grade of the animals possessing them far more nearly than 
those of any other organ or set of organs. If the brain, like other organs, 
develops by inteiligent use, it cannot be doubted that this relation of its 
development to grade is not accidental, but that grade structure is an ex- 
pression of its capacities, physical and mental. 
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