THE METHOD OF CREATION OF ORGANIC FORMS. 207 



nervous system, like others, develops in 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 pro- 

 portion 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 j)robable. Effort in the direction of thought is supposed 

 to convert heat into thought-force. Inasmuch as the more intelli- 

 gent animals possess the highest development of cerebral hemi- 

 spheres, it is highly probable that brain substance converts heat 

 into growth-force also, which produces tissue of its own kind pre- 

 cisely as muscle does. 



As different parts of the nervous centers subserve different 

 purposes, the development of these parts must proceed approxi- 

 mately under the influence of special kinds of effort and use. 

 Where, as. in the adult, heat is converted into growth-force in the 

 tissues to a 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 molecular change in its constitution. 



Now, we know physical and metaphysical peculiarities of 

 parents to be inherited by offspring ; hence, no doubt, the nervous 

 structure determinative of growth-force is inherited. This will 

 then control the localities of special conversion of heat, etc. 

 (from the mother), into growth-force, in accordance with the 

 structure of the parent, and the more decidedly, as its own in- 

 crease progresses. 



The result will be acceleration, or construction of tissues and 

 organs in excess of those of the parent, if the effort or use de- 

 voted to a nerve or organ be represented in the nerve-center of 

 the parent by a greater amount of force-converting tissue than is 



* The supposition that this and other forms of vital energy are derived from 

 metamorphosis of heat, was in accordance with views held at the time this essay 

 was written (1871). The much more probable hypothesis is that such energy is 

 derived from retrograde metamorphosis of nutritive material. (Ed. 1886.) 



